An American Airlines pilot died on his flight between Phoenix and Boston on Monday.

The official did not specify the cause of death beyond saying it was from an illness. The Onondaga County, New York, medical examiner's office said later Monday that an autopsy and preliminary toxicology tests "showed that the death was the result of natural diseases" without offering specifics, reports CNN.

"Whatever happened in the cockpit, none of us will know," passenger Frank Cacciola told CNN affiliate WCVB.

Cacciola was among the 147 passengers and five crew members aboard Flight 550 when it took off around midnight, heading from the Southwest to the Northeast. Their Airbus A320 plane was being operated by US Airways, which is now part of American.

The pilot fell ill somewhere en route, and the flight was diverted to Syracuse in upstate New York.

"Medical emergency, captain is incapacitated," someone in the cockpit told an air traffic controller, according to audio posted to LiveATC.net. "Request handling for runway."

Those on board knew something was wrong when a woman's "quivering" voice came over the intercom, alerting them the pilot was sick, passenger Peter McSwiggin told WCVB. Flight 550 then made a rapid descent, arriving in Syracuse at 7:13 a.m. ET, according to American's website.

Firefighters there responded to an unconscious person on the airplane, Syracuse Fire Chief Timothy Hess said. Hess was not sure if this person was the pilot, or what hospital that person went to, Reports CNN.

According to passenger Julia House, passengers waited for the body to be removed. They made it to Boston after noon, nearly five hours after their scheduled time.

Passengers expressed sadness about what happened and gratitude they arrived safely.

"If it wasn't for the copilot using a cool head," McSwiggin said, "it might have been more disastrous."