The FBI says $1.2 million in cash was discovered missing from a Swiss International Air Lines jet after it landed at John F. Kennedy Airport.

Agency spokesman J. Peter Donald says the plane arrived at the New York City airport on Monday. The FBI is investigating, but it remains unclear when the cash vanished.

No other details were immediately available, said Fox News. A source familiar with the investigation told the New York Post that representatives of the recipient opened the exterior of a cargo container, and found the smaller receptacle had been emptied with no signs of being compromised.

The weight of $1 million in $100 bills, for example, is estimated at 22 pounds, according to the US Treasury, the paper reported. This suggests that the theft might have occurred at the original departure point, the source added, the Fox News reported.

Investigators were also probing whether local mechanics, flight-crew members or other airport personnel had somehow gotten inside the inner container to swipe the money and then placed the smaller and now-empty cargo container back inside the larger one.

The airline says in an email that it could not confirm or comment due to the ongoing investigation.