A bomb apparently set to detonate at a certain altitude exploded yesterday in the cargo section of an American Airlines jetliner, filling the passenger cabin with smoke and forcing an emergency landing at Dulles International Airport.

None of the 72 passengers and eight crew members aboard American's Flight 444 en route from Chicago to Washington National Airport was reported injured in the dramatic mid-day incident. With passengers reportedly fumbling to use oxygen masks amid thickening smoke, the tri-jet Boeing 727 landed safely at Dulles at about 12:50 p.m.

Authorities said parts of an explosive device -- including a battery and an altimeter -- later were found in a mail container in the plane's cargo compartment. An altimeter is a barometric instrument designed to measure altitude. An FBI spokesman described the explosive package as 9 1/2 inches by 10 1/2 inches by 7 1/2 inches.

Authories said they had not established any motive for the explosive and knew of no previous incident in which a bomb sent through the mails was detonated aboard an aircraft.

"When the smoke started to get bad, I was wondering if we would ever make it," said Art Plotnik, a passenger aboard the flight. "It was as close a call as you can have." As the flight neared its end, Plotnik said, the smoke became so dense that it seeped through the passengers' oxygen masks.

According to the FBI, the pilot heard a thump aboard the aircraft a short time after it took off from Chicago at 11:20 a.m. Washington time. Nevertheless, the jet continued toward Washington. Then smoke began to seep into the passenger cabin, eventually prompting the pilot to make the emergency landing at Dulles.

Passengers hurriedly disembarked from the smoke-filled plane by a rear stairway, reportedly hugging each other in an emotional display of relief at the safe landing. The plane had been scheduled to arrive at National at 12:57 p.m. The captain, identified by an American Airlines spokesman as Donald M. Tynan of Boston, could not be reached yesterday for comment.

No one immediately claimed responsibility for planting the bomb aboard the plane. One knowledgeable source noted that if the bomb were sent through the mails, the sender would have no way of knowing which flight it would be placed aboard. The FBI said it was investigating the incident.

According to spokesman for the FBI and the National Transportation Safety Board, the explosion caused little damage to the plane other than blackening the inside roof of the cargo compartment. Officials said the aircraft's exterior was not pierced by the blast and that the plane's flight controls were unharmed. The NTSB spokesman said the explosive was apparently of "very low yield."

The passengers aboard the American Airlines flight included 12 persons who were on their way to a White House conference on libraries here. Plotnik, 42, editor of a Chicago-based journal called American Libraries, was in this group.

"About midway through the flight there was a sucking explosion and a loss of pressure," Plotnik recalled in an interview. "Everyone held their ears for a moment and then there was a funny smell -- a king of sweet chemical smell."

Shortly afterward, according to Plotnik, the captain announced over the loudspeaker that the trouble was caused by a "sticking valve." The pilot added, "It happens from time to time," Plotnik said.

About 20 minutes later, however, smoke began to seep into the passenger cabin near the ninth row, Plotnik recalled. A crew member announced that the plane would land soon and urged passengers to turn on their ventilation blowers, Plotnik said, but the smoke continued. "It just kept coming up thicker and thicker," he said.

"Very quickly, the whole fuselage filled with smoke. It was scary -- very scary," said Plotnik, who at the time was seated in the 14th row. Plotnik and other passengers near the front of the cabin were told to move to the rear of the plane, where the smoke was less dense. Oxygen masks descended from their ceiling housings, he said and stewardesses helped passengers put them on.

"No one really knew how to use them," Plotnik recalled. "People were -- jumping up and down and screaming for the poor stewardesses."

Then, Plotnik said, the captain announced that the plane would land at Dulles and the jetliner began a steep, rapid descent.

"We came to a screeching landing about 10 minutes later -- all the loose objects flew up to the front of the plane. They opened the back stair well," Plotnik recalled, adding, "I was the first one out of the plane."