Passed Out Pilot Twice Asked to Descend to Lower Altitude Small plane flew for more than four hours before crashing near Jamaica.

 -- The pilot of a small plane twice sought permission to fly at a lower altitude before he slumped over the controls, apparently unconscious as the plane flew for more than four hours before crashing into the sea off the coast of Jamaica.

But when asked by air traffic controllers if he wanted to declare an emergency, which would have made his request to descend a priority, the pilot said no, sources told ABC News. Shortly after that call, the pilot stopped responding.

Alarm over the plane that would not respond to radio calls prompted the Air Force to send two F-15 jets to shadow the plane. At one point one of the military pilots can be heard saying that the pilot of the turboprop was unconscious, but he appeared to be breathing.

"I can see his chest rising and falling. Right before I left, it was the first time we could see that he was actually breathing," the pilot said.

The turboprop was flown by Larry Glazer and his wife Jane Glazer of Rochester, New York. They were en route from Rochester to Naples, Florida.

Ken Glazer, the son of Larry and Jane Glazer of Rochester, told ABC affiliate WHAM that his parents were the only two people on board the plane.

The aircraft stopped responding to radio calls at 10 a.m. ET, according to a statement from the Federal Aviation Administration.

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The plane was flying at 28,000 feet at 10 a.m. when the pilot radioed air traffic controllers seeking permission to descend to 18,000 feet because of an issue with the plane, according to a federal official, an aviation source and live air traffic controller audio. Controllers cleared the plane to descend to 25,000 feet, but Glazer asked to go even lower, the sources said.

Controllers denied him permission to fly lower because of traffic below, but instructed Glazer to turn left so he could get out of traffic and then descend. Glazer made the left turn, but when controllers then gave him clearance to drop to 20,000 feet he had stopped responding, the sources said.

Aviation expert and ABC News consultant John Nance said the pilot’s requests to get to a lower altitude and his debilitation “clearly shows a progressive hypoxic situation,” a condition in which the body does not get an adequate supply of oxygen.

Nance said it wasn’t clear whether the pilot was incapacitated by the rapid depressurization of the cabin or fumes in the cockpit.

Lack of oxygen could have affected Glazer's judgement when he told controllers there was no need to declare an emergency just moments before he passed out.

The plane flew on down the Atlantic coast, crossed over Cuba and apparently ran out of fuel and crashed off the coast of Jamaica about 2:15 p.m.

The fighter jets followed the plane until it reached Cuban airspace. The jets were initially supposed to fly around Cuba’s eastern end and wait in international airspace to pick up the trail, officials said. But the planes instead returned to base to refuel.

Glazer was CEO and managing partner of Buckingham Properties and pioneered Rochester's downtown development.

Rochester Mayor Lovely A. Warren said the city would fly flags at half mast for the Glazers as well as Police Officer Daryl Pierson, who was shot in the line of duty.

"It is difficult to put into words how much Larry Glazer has meant to our community," the mayor said. "Larry worked hard to return a sense of vitality and excitement to our Center City. His efforts helped to lift our spirits and restore our sense of optimism. He has been a treasured friend and partner."

The FAA reported the plane to be a Socata TBM-700, while FlightAware said it believes the aircraft is a TBM-900.

A U.S. Coast Guard C-130 will look for the plane's wreckage, an official said. Jamaica said a search and rescue team had been dispatched to the area.