A convicted bomber and Serbian nationalist, who said he had explosives, hijacked a jetliner carrying 136 persons from New York to Chicago today. After releasing the plane's passengers, the American Airlines jet took off again and landed early this morning in New York City.

The hijacker had demanded freedom for a jailed Serbian nationalist comrade and a safe flight to Peru. American Airlines spokeswoman Mary Rose Noel said the plane took off just before midnight (EDT) with about 3 1/2 hours of fuel.

"We know where it's going. I can't say," said an American Airlines official who declined to give his name. "I know your problem, but I can't tell you."

The pilot of the Boeing 727, Capt. A. M. Mitchell, had refused to take the hijacker anywhere, but Noel said he and his co-pilot and engineer had stayed with the plane.

They were accompanied by the sole hijacker.

The passengers had been taken to a de-briefing area at the terminal after their release in late afternoon. Four female flight attendants and a male attendant also were released.

FBI spokesman George Mandich said the plane was returning to New York City, and would land at John F. Kennedy International Airport.

Noel had said earlier that the hijacker wanted to go to Peru. The vice consul of the Peruvian consulate in Chicago, Julio Vega, went to the airport, but no information was released on his activities.

U.S. Attorney Thomas P. Sullivan in Chicago identified the hijacker as Hikola Kavaja, 45. Kavaja had been free on bond and was on his way back to Chicago to be sentenced for his part in a 1975 bombing at the suburban Chicago home of a Yugoslavian consul to the United States. No one was injured in the bombing.

Sullivan said Kavaja wanted the release of Stojilko Kajevic, 43, a Serbian Orthodox priest who was previously sentenced in the same bombing. Two other persons were also convicted.

Kajevic was being held in the Metropolitan Correction Center, a federal jail in downtown Chicago, and was moved away from other prisoners after the hijacking.

In the hours after the passengers were released, there was confusion over whether there was only one hijacker or if he had an accomplice. Mandich said the hijacker had originally claimed to have an accomplice, and the FBI had difficulty finding firm evidence to the contrary.

"Somebody from the cockpit, I think it was the pilot, came back in the coach and told us there was this crazy guy sitting up front and he was going to hold the plane until his priest friend from Chicago was freed," said Jill Swanson, of the Chicago suburb of Schaumburg. Swanson said the hijacker told the passengers that he didn't want to hurt anyone and that he had three daughters of his own.

The jet had landed at 1:03 p.m. (EDT) and parked on a pad at a remote part of the airport that was quickly cordoned off.

During the standoff, people could be seen moving next to the plane, and a large yellow school bus, fire trucks and police cars were parked nearby. Two men were beneath the nose of the plane with communications gear hooked into an intercom system.

The nearest runway was closed, but otherwise normal operations continued at O'Hare, the world's busiest airport, where on May 25 an American Airlines DC10 crashed just after taking off, killing 273 in the nation's worst air disaster.

It was the fourth hijacking in the United States this year, and the second in two weeks. On June 11 a Delta Airlines plane was hijacked to Cuba while on a flight from New York to Fort Lauderdale.

American's Flight 293 had taken off from New York's LaGuardia Airport today at 11:16 a.m., and the pilot reported the hijacking at 12:29 p.m., when the aircraft was near Flint, Mich.

An American spokesman said standard security measures were in operation at the airline's loading area at LaGuardia. CAPTION: Picture, Passengers on American Flight 293 deplane after hijacked airliner landed in Chicago. AP