After months of scouring Africa, U.S. investigators have all but concluded that the 727 jetliner that mysteriously disappeared after departing from an Angolan airport in May crashed or was taken to a remote hangar to be stripped for parts.

U.S. intelligence officials had expressed fears that the 153-foot, 200,000-pound aircraft might have been stolen by terrorists for use as a weapon against Western targets in Africa. But an examination of satellite photographs and visits to dozens of African airfields failed to yield any evidence that it is on the continent, leading U.S. officials to largely discount the terrorism scenario.

Nearly two years after the Sept. 11, 2001, attacks, the case demonstrates that Western intelligence officials remain on hair-trigger alert for the possibility of terrorists' gaining access to large aircraft -- especially in Africa, where concerns over terrorism have risen dramatically in recent months, U.S. officials said. U.S. and other Western intelligence agencies still regularly unearth indications that the al Qaeda terrorist network remains interested in using aircraft to launch attacks.

As part of its search for the aircraft, the FBI has posted on its Web site a photograph and an announcement seeking information about Ben Charles Padilla, an airplane mechanic who is believed to have piloted the jet on its last known journey. The State Department is offering a reward for information about Padilla, and has placed his photograph on posters being distributed throughout Africa.

The mystery surrounding the jet began on May 25, when Padilla and another mechanic -- both of whom had been hired by the plane's owner -- entered the aircraft, which had been parked for months at Luanda International Airport in Angola. Just after 6 p.m., the jet rumbled down the runway, and despite protests from the control tower in Portuguese and English, it took off, never to be seen again.

The owner of the 1970s-vintage jet, Florida-based Aerospace Sales & Leasing Co., notified the FBI, and the government began its search. U.S. spy satellites snapped photos of African airports, and U.S. diplomats telephoned or visited dozens of airfields and aviation ministries. But no sign of the plane was found.

In late June, a Canadian pilot believed he had spotted the plane at the airport in Conakry, capital of the West African nation of Guinea, with its tail number sloppily covered by a new coat of paint. But when that plane was later tracked down in Lebanon, where its new owners had flown it, it was found to be a different 727.

A 727 sighted in Libya also turned out to be another aircraft. U.S. diplomats even checked airports in South and Central America, based on the theory that the plane could have crossed the Atlantic by refueling at a midpoint such as the Azores. That effort was fruitless as well.

One remaining theory, officials said, is that the plane was spirited away to an African hangar, where it could not be detected by spy satellites, and stripped for parts. Another is that it crashed, either in a remote forest, a deep lake or the Atlantic. Luanda is on Angola's seacoast, and pilots in the region say they often fly over the ocean for fear of drawing gunfire in the war-racked nation.

U.S. officials said they do not believe Padilla was involved in any wrongdoing involving the aircraft, so the theft-for-parts and crash scenarios could suggest he is dead. That has been the theory of Padilla's family members from the start.

Joseph Padilla of Pensacola, Fla., the missing man's brother, described the family as extremely close. He said: "[I]f he was alive and knew all this mess was going on, he would contact us." Another brother had e-mailed Ben Padilla in Africa days before the disappearance to inform him that their mother had suffered a massive heart attack, and Ben had replied that he would call her soon -- but the call never came.

"If he's still alive, the only way he wouldn't have contacted us is that he's being held captive someplace," Joseph Padilla said. "But if you were a terrorist, why go to the trouble of keeping someone captive for 24 hours a day? I hope I'm wrong, but I've got to face the reality that my brother likely is deceased."

Representatives of Aerospace Sales could not be reached for comment yesterday.

Ben Padilla and his Congolese assistant, John Mikel Mutantu, had been hired by the firm in the spring to repair the plane, ensure its airworthiness and straighten out disputes with Angolan officials who had grounded it.

Angolan officials said tens of thousands of dollars in landing and parking fees were owed on the jet, and that its interior configuration violated air safety standards because the seats and galleys had been removed to allow the installation of huge internal fuel tanks. The plane had been leased at various points to deliver fuel to remote diamond mines around Angola.

Angolan airport officials told U.S. representatives that Padilla and Mutantu were the only people to board the plane that day. Mutantu was not a pilot, and Padilla, while licensed to fly a 727, was an unskilled pilot, people who know him said.

One pilot who had flown the plane recently and who requested anonymity said it was in such poor shape that on one outing it inexplicably lost cabin pressure. He also said its emergency locator beacon -- which would help pinpoint its position in a crash -- was inoperable.

U.S. officials said a number of individuals and companies had either leased or sought to buy the jet in recent months, and that some of them, including a few who had staked a claim to the 727, had engaged in shady business deals.

Aviation sources with direct knowledge of events said some of the parties involved had discussed simply removing the plane without Angolan officials' approval or paying the debts. It is unclear whether the Angolans were ever paid.

"How we'll ever really know what happened to it," one U.S. official said, "I can't imagine."