An investigation is under way into how a gun carried by a US Airways pilot was discharged during a flight.
No-one was hurt when the gun went off as the plane was preparing to land at Charlotte, North Carolina, on Saturday.
A hole in a cockpit wall apparently caused by the shot is visible in photos obtained by AP news agency.
Under a programme implemented after the 9/11 attacks, US airline pilots are allowed to carry guns on domestic flights following a training course.
The gun discharged just before noon on Saturday aboard Flight 1536 from Denver to Charlotte, as the Airbus A319 plane was at about 8,000 feet (2,440m) and was approaching to land.
The plane was sequestered and the Transportation Security Administration (TSA) is looking into the incident, spokesman Dwayne Baird said.
He said the airline was co-operating and that the FBI would also probably be involved.
The pilot has been put on leave pending the outcome of the investigation.
Mr Baird said he believed the 124 passengers on board at the time were not aware of the incident.
Aeronautical experts disagree on how much risk there is that such a stray bullet could bring down an aircraft.
Under the TSA's Federal Flight Deck Officer programme, pilots may apply for a licence to carry a gun on board for domestic flights.
They must undergo a week-long training course and psychological tests.
Since April 2003, about 5,000 flight deck officers - captains or first officers - have been authorised to carry weapons, Capt Bob Hesselbein, chairman of the Air Line Pilots Association's National Security Committee, told the Associated Press.
All pilots who qualify from the programme are said to carry the same weapon - a .40-calibre semiautomatic H&K USP - which experts say is extremely unlikely to go off on its own.