A police officer declared a hero Monday night for defusing a bomb seemingly meant for Turkish Olympic athletes admitted today that he planted the device.

The confession by Officer James W. Pearson, obtained after he failed an early morning lie- detector test, put a startling, closing twist to what had been the only known serious security threat to the 1984 Summer Games.

And in the only known defection related to the games, Romanian journalist Vladimir Moraru, 38, who had been hired as an interpreter by The San Diego Union, was granted political asylum.

Pearson, 40, was arrested for possession of a destructive device and jailed. Bail was set at $60,000.

Los Angeles Police Chief Daryl Gates said that Pearson, a nine-year department veteran, built the makeshift bomb in his van four days ago after failing to get desired assignments in the department's metropolitan division.

"He wanted to do something to cause them to take notice," Gates said. "We have all noticed him at this point."

Los Angeles Mayor Tom Bradley called it "unfortunate" that one officer "could in any way detract from the tremendous job done by all the others." Police officials said the games were so uneventful that they may refund up to $1 million in security costs to the local Olympic committee.

Police spokesmen said Monday night that Pearson had spotted a ticking pipe-bomb in the wheel well of a bus carrying athletes' luggage and had pulled out one of its wires and carried it 60 yards across the tarmac of Los Angeles International Airport. Gates rushed to the airport to hail the deed as a "helluva courageous act."

This morning a caller to the Paris bureau of the Associated Press said the Armenian Secret Army for the Liberation of Armenia claimed credit for the bomb attempt. Armenian terrorist groups blame Turkey for the massacre of hundreds of thousands of Armenians in 1915 and have assassinated more than two dozen Turkish officials in the last decade, including the Turkish counsel general in 1981.

But at a news conference today, Gates said he and department investigators began to doubt Pearson's story Monday night "because of the very careful way those buses had been examined before they left UCLA the University of California at Los Angeles ," where the Turkish athletes had been living in one of the two main Olympic villages.

Before the alleged discovery of the device at 5:45 p.m., Olympic security officials had examined the luggage bus and an accompanying bus carrying 52 Turkish athletes and watched for attempts to approach the buses before they reached the airport.

A police bomb squad retrieved the 5-by-8-inch shiny black device from where Pearson dropped it and took it to a safe area for disarming. Flights were delayed and about 6,000 people were evacuated from three airport terminals as a result of the incident and two subsequent bomb threats.

A spokesman said Pearson first volunteered to take a lie-detector test after investigators noted discrepancies in his account of the incident and learned that he had experience with explosives.

Police commander William Booth said that about 2 a.m. today, when the polygraph indicated that Pearson's story was false, Pearson told investigators that he had planted the device. A second polygraph test confirmed this.

Gates said Pearson insisted that he had rigged the device so it would not explode, but investigators determined that it contained enough smokeless gunpowder to be covered by felony statutes.

"This is particularly sad," Gates said. "He had a remarkable record. He had numerous commendations. We are very sad that he has chosen to do this."

Immigration officials said Moraru contacted the FBI Monday night and filed for asylum early this morning. His application was granted by State Department and local Immigration and Naturalization Service officials within hours, said John Belluardo, an INS spokesman.

Belluardo would not discuss Moraru's reasons for defecting. Moraru has a wife, a 5-month-old daughter and a brother in Romania. Belluardo said Moraru "is free to relocate and to work anywhere he wants."