( 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 does n't mean someone shot at the plane , said Amy Thorenson , a spokeswoman for the FBI in Charlotte , North Carolina . Investigators do n'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 .
NEW : Airline : `` We 're not speculating '' on what caused the hole . A pilot discovered a small hole in the exterior of the aircraft . An FBI spokeswoman says the hole is bullet-sized , but not necessarily from a gun . The plane was carrying about 100 passengers and crew members when it went down at around 12:30