(CNN) -- FBI investigators were working Tuesday to discover what caused a small hole in the body of a US Airways jet.
A pilot discovered the hole Monday while checking the aircraft before a flight, the airline said.
The puncture in the Boeing 737 airliner is the size and shape of a bullet hole, but that doesn't mean someone shot at the plane, said Amy Thorenson, a spokeswoman for the FBI in Charlotte, North Carolina. Investigators don't yet know what could have caused the hole, she said Tuesday morning.
The airline declined to comment.
"We're not speculating on the hole and what caused it," airline spokeswoman Valerie Wunder said.
The aircraft had flown to Charlotte from Philadelphia and was being prepped for another flight when the pilot discovered the hole above a passenger window toward the back of the plane, according to airline officials. The airline pulled the plane from service and called in the FBI.
It's unclear whether the hole appeared before or during that flight or while the airliner was on the ground in Charlotte, but investigators and the airline were operating under the assumption that it happened in flight, Thorenson said.
The airline called in the FBI, which Thorenson said has jurisdiction over anything that happens to a commercial passenger airliner in flight.
The airline is also investigating, Wunder said. The airplane remained idle Tuesday, she said.
The plane -- a 737-400 designated as flight 1161 from Philadelphia -- had landed without incident at Charlotte-Douglas International Airport at about 4 p.m.
The plane holds 144 passengers, according to the US Airways website.
It was not immediately clear how many people were aboard that flight, but all of the passengers were accommodated on other flights, a US Airways spokeswoman said.
CNN's Rich Phillips contributed to this report.