United Airlines Flight 3613 was grounded at Burlington International Airport in Vermont on Sunday after a bomb threat was found on a note left in a restroom. The 4:30 p.m. flight carrying 65 passengers from Newark, New Jersey, was held for hours as officials searched for any sign that the threat was real.

NBC reports that airport director of aviation Nic Longo said at a press conference that about 20 minutes before landing, the flight crew told Burlington’s control tower that there was “a threat to the aircraft.” Longo gave no details about the threat, but one NBC affiliate got a recording from the pilot of 3613 stating that a “flight attendant just came up and said a passenger had found a note … saying there was a bomb on board.”

Under emergency response protocols, the aircraft was sent to a special location for the situation to be assessed by the Vermont State Police Explosive Ordnance Disposal Unit and additional local and federal authorities.

Passengers described K-9 units sweeping the plane, with one describing the experience as “scary”. Another, Phil Baruth, said it was “unfair” that the threat “ruined everyone’s days”.

“Obviously someone is a little crazy,” Baruth said. “On the other hand, the authorities here handled everything perfectly and I never felt unsafe for a moment.”

This is just the most recent threat of its kind to disrupt flights since late February. a threat on Feb. 17, an American Airlines flight diverted in Texas and a Spirit Airlines flight was evacuated from a New Hampshire airport on February 11 due to a bomb threat.

In Burlington, investigators were unable to find a credible threat and there were no arrests, according to NBC. The FBI is leading an ongoing investigation into the incident.