The Reagan administration, flush from its reelection landslide, passed the word five days ago that it had "credible evidence" that a Soviet arms-carrying ship was bringing advanced fighter jets to Nicaragua and warned that such a move would not be tolerated.

The warning brought heated denials from Nicaragua's leaders and created a sense of panic in the capital of Managua, where officials said they feared that the United States was seeking a pretext for military action.

By week's end, tensions appeared to subside as U.S. officials said they now believe that the ship, the Bakuriani, was not carrying MiGs, although they still could not be sure.

What remained were the questions of who created the crisis and why.

A combination of factors may have been responsible -- concern in the U.S. intelligence community, media overreaction to that concern, Nicaraguan overreaction to U.S. press reports and a deliberate attempt by the newly reelected administration to send Congress and the Nicaraguans a message.

One specialist in Central American policy said that an administration official had described the episode as an intentional exercise in "perception management" by the government.

"They've had to restrain themselves in the last four years, but no holds are barred now," the expert said. "They wanted to make very clear to the Nicaraguans at the very beginning of the administration what the rules are."

Another source close to the administration said officials wanted to persuade Congress to resume funding the CIA-created rebel army that is seeking to overthrow the Nicaraguan government. This source said that the effort had gone somewhat awry, with too much focus on the MiGs, and by Friday, administration officials were stressing instead the dangers of an overall arms buildup in Nicaragua.

But a senior administration official denied any effort to create a crisis.

"It was you guys the press who were sending the signals," the official said. "And it was the arrival of the ship that set the timing."

Secretary of State George P. Shultz said yesterday that whoever leaked the story about the MiGs "engaged in a criminal act in my opinion."

And State Department spokesman John Hughes noted that he had told reporters since Wednesday that officials were unsure about the ship's cargo.

"We have consistently urged everyone to take a cool, calm, clinical look at this story," he said. "I do not think the administration could fairly be accused of starting a scare or seeking to publicize the situation."

But while Hughes and White House spokesman Larry Speakes urged caution during the week, other senior administration officials appeared to stoke the fires in not-for-attribution interviews. By the time the episode ends, one official said, he fears that the result will be to stimulate Nicaraguan paranoia and prompt an escalation of that nation's military buildup, opposite what the administration hoped.

The story of the Bakuriani starts in the Black Sea, where the ship began its journey more than a month ago.

U.S. officials say that intelligence photographs showed a dozen crates of the type that normally carry MiG21 fighter jets sitting on a dock next to the ship.

No one saw the crates loaded onto the ship, but subsequent photographs showed the crates and the Bakuriani gone. Officials said it was logical to suspect that the crates were on the ship.

Intelligence officials routinely monitored the freighter's journey around South America and past Peru.

On the morning of Election Day, when it appeared certain that the ship was headed for the northwestern Nicaraguan port of Corinto, Shultz was informed.

U.S. officials had warned Nicaragua many times before, beginning in 1981, not to import MiGs. There had been reports of crates in Cuba, pilot training in Bulgaria, runway construction at Punta Huete, Nicaragua.

But officials let it be known that they were treating this episode with particular seriousness. Shultz had his undersecretary of state, Michael Armacost, inform the Soviets of U.S. concern, and Ambassador Arthur Hartmann carried the same message in Moscow.

The Soviets assured U.S. officials that there was nothing to worry about on the ship, officials said. The ship, shadowed by a U.S. frigate and U.S. naval planes, lingered outside Corinto for some time and docked Wednesday.

The first public mention of U.S. concern came Tuesday evening on CBS, which reported that officials believed that the ship "may be carrying" MiG21s.

"If those planes are on the ship, one U.S. option being considered is a surprise attack to destroy them," CBS reported.

NBC followed with a report on the ship later that evening, and several newspapers, including The Washington Post, reported the next morning that senior officials were concerned about the ship's cargo. The Post also reported that Speakes was cautioning reporters "not to jump to any conclusions about which kind of aircraft are aboard."

Nicaraguan Foreign Minister Miguel D'Escoto said it was "a total lie" to suggest that MiGs had arrived or were on their way. Nicaraguan officials were up much of the night, fearing that U.S. officials were setting the stage for a Grenada-style invasion.

President Reagan, in a Wednesday news conference, declined to rule out the use of force.

"We're keeping a careful watch," he said.

"As I say, I'm not going to comment on what might follow, or what our procedure might be."

Other officials speculated on Nicaraguan motives for importing MiGs and discussed how dangerous such a step would be for other Central American nations. They said they would consider options for "neutralizing" the planes if they were delivered.

Spy planes flew over Nicaragua, apparently causing sonic booms that increased Nicaraguan anxiety. The frigate USS Paul steamed just beyond Nicaragua's territorial waters.

Long-scheduled naval and Army exercises were taken by the Nicaraguans as signs of impending military action. Reporters, who had been told by Speakes the night before the Grenada invasion that the idea of such an action was "preposterous," were reluctant to take Pentagon spokesmen at their word when they said the exercises were routine.

Meanwhile, the Bakuriani was being unloaded -- helicopters, small arms, ammunition, vehicles, small boats. No MiGs appeared.

Officials said it was possible that MiGs remained on the ship and would never be unloaded. They said it was possible that the Soviets had deliberately confused U.S. intelligence or that U.S. assumptions simply were incorrect.

In any case, administration officials began talking more of the dangers of Nicaragua's overall arms buildup, its antiaircraft missiles and radars, its tanks and helicopter gunships, rather than just the MiGs.