The FBI is investigating a bullet hole found in a US Airways jet's fuselage. The hole was detected by a pilot during a pre-flight inspection.
Law enforcement officials said a .40-caliber bullet penetrated the skin of the plane and was found inside, NBC News has learned. However, from the trajectory of the round, officials think it entered the plane from above.
Investigators assume the incident was accidental, and said there isn't evidence suggesting the jet was targeted. They don't know when or where the round was fired.
Michelle Mohr, an airline spokesperson, said US Airways called in the FBI because "it looked like it could have been" a bullet hole.
"It's very small," Mohr said. "This pilot has a heck of an eye."
The hole near the rear of the plane was found after it arrived at Charlotte Douglas International Airport on Monday from Philadelphia. The airline said there were no apparent problems during the flight. Passengers who were going to be on the plane for its next flight were put on other flights, the airline said.
Mohr said the hole in the Boeing 737 is being repaired and the plane will return to flying on Wednesday. The plane seats 144 people, although it wasn't clear how many were on the flight from Philadelphia on Monday.
Transportation Safety Administration spokesman Jon Allen said his agency is cooperating with the FBI investigation. Charlotte airport director Jerry Orr and the FBI did not return messages Tuesday.
Federal Aviation Administration spokeswoman Kathleen Bergen said Tuesday that her agency also will ensure the aircraft is airworthy before it's put back in service, although that doesn't mean an FAA inspector will physically check the plane, she said.
"The airline has the mechanics. They have the manuals. They have skills and capability and authority to do those repairs. They do the required record keeping and then we can inspect that if necessary," she said.
Besides bullets, holes can be caused by metal fatigue or lightning. In October, a cabin lost air pressure when a hole ripped open in the fuselage of an American Airlines flight from Miami bound for Boston. In that case, the pilot had to make an emergency landing.
Information from The Associated Press is included in this report.