(CNN) -- A hole in a US Airways jet that landed in Charlotte, North Carolina, was caused by a bullet that pierced the passenger cabin, three government sources told CNN Tuesday.

Officials believe the bullet was fired in Charlotte, after passengers had exited the aircraft, one source said. The hole was discovered after the Boeing 737-400 landed Monday.

The sources said a bullet has been recovered inside the plane.

"We do not believe its terrorism related," said one of the government sources. "It appears to be a random event. We do not believe the plane was targeted. No one heard the bullet fired."

An investigation into who fired the shot into the aircraft has begun, said multiple government sources.

Flight 1161 from Philadelphia landed safely at Charlotte-Douglas International Airport about 4 p.m. Monday. The plane 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 jet from service and called in the FBI.

The plane holds 144 passengers, according to the US Airways website. It was not immediately clear how many people were aboard the flight. All of the passengers on the next flight were accommodated on other planes, a US Airways spokeswoman said.

"We've released the plane back to US Airways last night," after completing their investigation, FBI spokeswoman Amy Thoreson said Tuesday.

But the airplane remained grounded Tuesday while US Airways performed its own investigation, said company spokeswoman Valerie Wunder.

Before the plane can be put back into service, it will have to be inspected by the Federal Aviation Administration for flight certification, according to agency spokeswoman Kathleen Bergen.

CNN's Rich Phillips contributed to this report.