American aviation officials are investigating after an Air Canada flight from Montreal landed on a San Francisco runway after being told not to.

Federal Aviation Administration spokesman Ian Gregor says Air Canada flight 781 was inbound to San Francisco International Airport on Sunday night and was initially cleared for landing.

But Gregor says air traffic control later instructed the Air Canada crew to abort the landing because the controller wasn’t sure whether a previous flight would have completely cleared the runway before the Air Canada jet reached it.

He says the Air Canada crew did not acknowledge any of the controller’s instructions over the radio, so a supervisor then used a red light gun to alert the crew not to land.

But again, the crew didn’t respond, and Gregor says the flight landed immediately.

After landing, the Air Canada crew told the control tower that they had a radio problem.

Gregor says that a radar replay showed the previous arrival was clear of the runway when the Air Canada flight landed.

Representatives from Air Canada did not immediately respond to a request for comment.