When Korean Air Lines Flight 007 left Anchorage Wednesday after a routine refueling stop, it began a strange, mysterious odyssey that ended in death not only for the 269 people on board but for hopes of relaxation soon of tensions between the United States and the Soviet Union.

Relatively little is known about what happened and why in the 4 hours and 26 minutes between the departure of the Boeing 747 from Anchorage en route from New York to Seoul, and the time the United States says a Soviet SU15 jet fighter shot it down over the Sea of Japan, about 310 miles off course.

In the hours following the disappearance of the jetliner, there were reports that it was missing, that it was safe on a Soviet island and, finally, the reality: that it was lost with all its passengers. Confusion has dominated efforts to reconstruct the tragedy--from the Japanese, who were in the best position to monitor what had happened, and the Koreans, who gave conflicting information to anxious friends and relatives of the passengers, to the highest levels of the U.S. government and possibly the Kremlin, which has an international crisis on its hands.

Some details are beginning to fall into place as the western allies' intelligence network assembles the pieces of the mystery from chips and electrons and magnetic tapes and radar antennae. But the answers to the two most obvious questions may never be known:

Why was Flight 007 hundreds of miles off course, clearly inside territory marked on every aviator's map as dangerous for intruders?

Why did someone in the Soviet Union decide to shoot down an unarmed commercial jetliner?

Here, from a variety of sources, is an account of what had happened through Friday night, when President Reagan cut short his vacation in California and returned to Washington to meet with the National Security Council. All times are Eastern Daylight, unless otherwise noted.

At 10 a.m. Wednesday, Flight 007 left Anchorage after a refueling stop, climbed to its assigned altitude of about 31,000 feet and proceeded to R20, one of five internationally approved "great circle" routes across the north Pacific for air traffic between Anchorage and the Far East.

In less than an hour and about 200 miles from Anchorage, the airliner left the range of U.S. air traffic radar. But it made its five expected progress reports to Anchorage before switching its radios to controllers in Tokyo. There was no reason for alarm; the plane was using an extraordinarily reliable on-board navigation system.

But at some point in the two hours after takeoff, Flight 007 strayed north and west of its assigned course and penetrated Soviet airspace. At noon Wednesday (1 a.m. Thursday, Soviet time), the plane was picked up by Soviet radar over the Kamchatka Peninsula, a part of the Soviet Union containing sensitive missile installations. A number of Soviet jet fighters scrambled to find the intruding aircraft.

Thus, the radio reports that Flight 007's crew was making about its progress were reassuring, but apparently inaccurate, possibly because of an error in the programming of the airliner's in-flight navigation computer.

At 1:10 p.m. Wednesday (2:10 a.m. Thursday, Soviet time), Flight 007 called Tokyo's Narita air traffic control center and reported, "We passed safely south of Kamchatka."

But it is now apparent that the airliner had instead crossed the Kamchatka peninsula and was flying over the Sea of Okhost.

An hour later, at 2:10 p.m. Wednesday (3:10 a.m. Thursday, Soviet time), a Soviet SU15 fighter pilot reported visual contact with the Korean jet. Normally, a fighter intercepting another airplane would overtake it, then fly in formation with it, matching its speed to that of the intercepted plane.

Fighter pilots usually are trained in identifying other aircraft by silhouettes and other means. There is a disparity between U.S. officials and Soviet statements about whether the Korean Air Lines jetliner had its normal navigation lights turned on. But U.S. experts believe that, even in the early morning, enough light was probably available to see that it was a passenger plane.

Nonetheless, radio traffic monitored by the Japanese, which U.S. officials have in a transcript covering the climactic half hour of the 2 1/2 hours the Korean Air Lines plane was being monitored by the Soviets, shows it was always referred to by the Soviet fighter pilot as "the target."

This led one senior U.S. official to tell The Washington Post that the Soviets are guilty either of "enormous callousness" in shooting down a plane they knew to be an airliner or "incredible incompetence," possibly mistaking the uniquely shaped 747 for some kind of surveillance aircraft.

There also is dispute about whether Soviet fighter pilots warned the Korean Air Lines pilot that his plane might be attacked or whether any of them employed the internationally understood signal of flying in front of the airliner and rocking the wings of their jet fighters to indicate the intercepted plane should follow them to the ground.

The Soviets said in a statement released two days later that warnings were given and that warning shots were fired as a last resort.

U.S. officials said there are some "obscure" portions of the tapes from which a deduction can be made that a Soviet fighter may have circled in front of the Korean Air Lines plane and may have wiggled his wings.

But they said there are no known radio transmissions from the Korean Air Lines pilot to indicate that he suspected his airliner was in danger.

At 2:23 p.m. Wednesday (3:23 a.m. Thursday Soviet time), Flight 007 told Tokyo's Narita control that it had reached an altitude of 35,000 feet.

Just three minutes later, according to a statement by Secretary of State George P. Shultz on Thursday in Washington, "The Soviet pilot reported he fired a missile and the target was destroyed."

An "incomprehensible radio noise" was received at Narita from Flight 007 before it disappeared from radar screens. That took about eight minutes, according to Shultz' statement, which would indicate that the plane may have continued flying, seriously disabled, for at least a while. If it had lost a wing or suffered significant damage to a major control surface, it would have plunged to the ocean in less than a minute, according to experts.

There has been no complete explanation about why, if this much is known from military intelligence monitoring, no apparent attempt was made to warn Flight 007 that it was off course and in peril, tracked by eight Soviet fighters. Military and intelligence radar facilities are not normally used for air traffic control purposes, however, and U.S. officials said their account has been reconstructed and the transcript of the monitored conversations made hours after the event.

Confusion followed the disappearance of the plane. There were television news reports of a lost airplane, then a comforting report that the flight had landed safely on the Soviets' Sakhalin Island. Most television news viewers here went to bed with that report and awakened to the truth on the morning news shows.

At 8:30 p.m. EDT, six hours after the Korean Air Lines plane disappeared, Frederic N. Smith, administrative assistant to Rep. Larry McDonald (D-Ga.), who was aboard the airliner, called the State Department operations office and was told that the U.S. Embassy in Seoul was reporting the plane in the water 120 miles southeast of Hokkaido.

It was eight hours after the shots were fired, at 10:30 p.m. (7:30 p.m. in California), before the first reports about the Korean Air Lines plane reached President Reagan at his ranch near Santa Barbara, where he was vacationing. Reagan was called by National Security Adviser William P. Clark, who was elsewhere in California.

According to White House spokesman Larry Speakes, the first reports were not clear. "It was regarded as a missing aircraft for a long period of time," he said, "and we were monitoring, trying to make assessments."

At the time of the first call to the president, Speakes said, "There were conflicting reports. The president was given the extent of our information on it and all viewpoints on it . . . ."

Between 10 p.m. and 11 p.m., someone in the State Department called McDonald's wife, Kathryn, in Georgia, according to a relative, and told her the plane had been forced down. At 10:30 p.m., a State Department special operations group was formed with officials from the Pentagon, the White House and intelligence agencies.

In Washington, McDonald aide Smith was briefed at the Pentagon and told there was "evidence of a shoot-down, which looked authentic," according to Smith. He said he was read intercepts that had been sent by telecopier from Japan and was told to tell the news media that "no U.S. source is able to confirm that passengers are alive," as was then being reported. The Pentagon estimates Smith was briefed at 1 a.m. EDT.

At midnight, Richard Burt, assistant secretary of State for European affairs, had called Soviet charge d'affaires Oleg M. Sokolov and asked for an explanation of the plane's fate. In Moscow, the U.S. Embassy made a formal inquiry to the Soviet Foreign Ministry. The Soviets, Burt said, provided no information.

Clark called Reagan back at 1:30 a.m. Thursday (10:30 p.m. Wednesday in California). "It was a little clearer, and we had some speculation, and I would judge fairly hard information at 10:30, based on intelligence reports," Speakes said. "But then again we were still assessing, as we did throughout the night." That was the last call made to Reagan Wednesday.

About 3 a.m. Thursday (midnight in California), White House officials received information the plane had been downed. "But we still did not have, that it had disappeared from radar," Speakes said. "So we still did not have a detailed assessment of why it was there. We did not know whether it had been forced down or whether it had been actually fired on."

The White House is extraordinarily sensitive to questions about when Reagan is informed of crises while at the ranch because of the criticism he received in the news media after the Libyan plane incident in 1981. Reagan, who was the staying at the Century Plaza Hotel in Los Angeles, was not awakened by White House counselor Edwin Meese III after U.S. jet fighters shot down Libyan jets.

At 2:15 a.m. Thursday in Washington, Shultz spoke by telephone with Clark, apparently clearing with Clark the statement Shultz planned to make on national television. But Shultz did not talk to the president.

Reagan was told at 7:10 a.m. in California that the Korean Air Lines plane definitely had been been shot down. Speakes said he could not cite a precise moment when the U.S. government reached that conclusion.

Shortly after 10 a.m. EDT, with the nation's news organizations already quoting sources saying the plane had been shot down, Shultz held the news conference in which he detailed for the first time the intercepted radio traffic that led U.S. officials inexorably to the conclusion that Korean Air Lines Flight 007 had been shot down by the Soviets.

Shultz was questioned at the news conference about whether he had spoken to the president. He said he had not. He then talked by telephone with Reagan for about 15 minutes at 11:33 a.m. EDT.

In California, several of Reagan's aides gathered at the Santa Barbara Biltmore to review the intelligence information and suggest an appropriate statement for Reagan. Meese and Speakes worked out the statement and called Reagan for his approval.

At 10:05 a.m. California time, Speakes stepped to the podium at the press center in the Santa Barbara Sheraton.

"I have two statements, the first on the Korean Air Lines incident," he said. "The president is very concerned and deeply disturbed about the loss of life aboard the Korean Air Lines flight overnight. There are no circumstances that can justify the unprecedented attack on an unarmed civilian aircraft. The Soviet Union owes an explanation to the world about how and why this tragedy has occurred. At the direction of the president, the secretary of State is demanding an immediate and full account of this tragic incident from the Soviet Union . . . . He Reagan is being kept abreast and will be kept advised throughout the day as the assessments proceed by officials in Washington."

At this point, Speakes was interrupted and asked if Reagan was going back to Washington. He ignored the question and read a statement on the Middle East. Asked again if Reagan was going back to Washington, Speakes answered, "There are no plans for the president to return to Washington earlier than anticipated."

Speakes walked away from the podium and then came back to take questions. He announced, as he does every day in California, what Reagan intended to do that day: "The president, as usual, is planning at horseback ride this morning and will generally work around the ranch in the afternoon. The weather there is as it is here, sunny and warm."

Speakes was obviously sensitive to questions about whether Reagan was being kept fully informed. "Well, as we've discussed many, many times here, that the president, here at the ranch, from the first day he landed at the ranch, as president, has every facility, every capacity, every capability to do, perform any function that he could perform in Washington. He essentially has at his disposal every initiative that he would care to take, from the most far-reaching to the most simple initiatives, and at the same time, he has at his disposal the same information that he would receive sitting in the Oval Office of the White House, and he is fully capable of having detailed discussions with his advisers, and he has done so . . . ."

As questions continued about whether and when Reagan was informed, Speakes became exasperated. "The point is, that we did not know, did not have final, firm confirmation and a willingness to go public with it until shortly before the president was informed of it," he said. "As soon as they were certain on it, they informed the president of it. Now, that was 7:10 a.m. for you tick-tock fanatics."

At Speakes' afternoon briefing, which began at 2:33 p.m. California time, he read a long statement from the president, saying, " . . . Words can scarcely express our revulsion at his horrifying act of violence. The United States joins with other members of the international community in demanding a full expanation for this appalling and wanton misdeed. The Soviet statements to this moment have totally failed to explain how or why this tragedy has occurred. Indeed, the while incident appears to be inexplicable to civilized people everywhere . . . ."

When a reporter again asked if the president was going back to Washington, Speakes announced that Reagan had decided to cut short his vacation and return Saturday for meetings with the National Security Council and congressional leaders.

Speakes said Reagan wanted to "consider ways in which the United States may join with Korea and Japan to express effectively the disgust that the entire world feels at the utter barbarity of the action of the Soviet government in shooting down an unarmed, non-military passenger plane and their refusal thus far to acknowledge either the responsibility or their sorrow for this action."

Asked what made Reagan change his mind about returning to Washington, Speakes dodged the question, saying that Reagan had been reviewing the situation and "will utilize tomorrow Friday in an effort to continue assessments."

But Thursday wasn't over, even though reporters and White House staff were getting punchy. At 6:15 p.m. California time, Speakes called reporters back to the briefing room to announce that Reagan had decided to return to Washington Friday instead of Saturday, because, he said, it would be easier to get the National Security Council together then. Asked if the switch was a public relations effort, Speakes said, "No."

Meanwhile, officials in Washington Thursday were moving on technical as well as diplomatic fronts. Federal Aviation Administration Administrator J. Lynn Helms and James H. Burnley, general counsel of the Transportation Department, met with State Department officials to talk about air traffic control.

The air route from which Flight 007 had strayed comes within 17 nautical miles of Soviet airspace. It was decided that for precautionary reasons it should be closed temporarily, and it was. At 4:30 p.m., Helms told the Anchorage air traffic control center not to grant clearances to flights seeking to use the route; the United States controls access to the route from the north, so that action closed it. Four parallel routes remain open.

U.S. officials also were concentrating on the two main questions. Considerable effort was spent by administration officials assuring reporters that the Soviet pilot had not fired without instructions from the ground and that the decision to shoot had come from Moscow. The level at which that decision was reached has not been established.

Early Friday, the Soviet news agency Tass released a tough statement, charging that the Korean airliner had been on spy mission over its territory and claiming that only tracer rounds had been fired at the aircraft after it ignored interception attempts and before it departed Soviet surveillance.

U.S. officials had anticipated the charge that the airliner was a spy plane, double-checked through intelligence sources and said later that it was "preposterous."

Aviation sources said the Japanese and the Soviets were searching the Sea of Japan to recover the "black boxes," the flight recorders that, if found, could help explain how the Korean plane came to be so far off course.

Questions also were raised about the competence of Korean Air Lines crews generally, since the most plausible explanation for Flight 007's strange route is that its navigational computer was misprogrammed by a human and since another Korean Air Lines jetliner had strayed 1,000 miles into Soviet territory in 1978 before being forced down on a frozen lake. Two people were killed.

"My problem with that careless airline theory is that it doesn't work," said a senior FAA pilot. "It's just like any other airline. When you start talking about those kinds of things you have to talk about individuals. Every airline has a spectrum of skills and dedications, and we still haven't automated the whole damn flight."

Meanwhile Friday, Reagan flew by helicopter from his ranch to Pt. Mugu Naval Air Station near Oxnard, Calif. and landed at 9:35 a.m. California time. With his wife, Nancy, at his side, Reagan walked to a podium in front of Air Force One and read a statement:

"What can be said about Soviet credibility when they so flagrantly lie about such a heinous act? What can be the scope of legitimate mutual discourse with a state whose values permit such atrocities, and what are we to make of a regime which establishes one set of standards for itself and another for the rest of humankind?"

On the flight back to Washington, a senior official told reporters that Reagan isn't going to use arms-control talks as retaliation for the airplane incident, despite some speculation to that effect.

The National Security Council meeting at the White House Friday night was crowded. It included Reagan; Vice President Bush; Adm. Dan Murphy (Bush's chief of staff); Shultz; Treasury Secretary Donald T. Regan; Defense Secretary Caspar W. Weinberger; Attorney General William French Smith; Burnley; Helms; Budget Director David A. Stockman; CIA Director William J. Casey; Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff; Charles V. Wick, head of the U.S. Information Service; Meese; Clark; James A. Baker, White House chief of staff, and Michael K. Deaver, deputy chief of staff.

According to Speakes, Reagan received a "detailed update," including "a review of the factual information" and the Soviet response. Reagan also received a set of options that focus on a "measured" response, Speakes said, in which U.S. officials would act in concert with Japan and South Korea.

Speakes said Reagan "emphasized that as of this moment we have received no satisfactory response from the Soviet Union for their outrageous conduct."