Law enforcement responded Sunday to the Burlington International Airport in Vermont.
South Burlington police confirmed they were assisting Burlington police with a "Phase 5" at the airport, the highest emergency alert in these situations.
NBC affiliate WPTZ reports the Vermont State Police bomb squad responded to the airport, at the request of the Burlington Police Department, to investigate an "unlawful threat" on a plane.
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Police said the threat was made on United Airlines flight 3613, which took off from Newark, New Jersey, at 3:14 p.m. Sunday and was bound for Burlington. Officials said about 20 minutes before landing in Burlington, the pilot was made aware of a threat that was found inside of a bathroom on the plane.
The pilot notified Air Traffic Control officers, prompting the large emergency response.
The flight landed in Burlington around 4:30 p.m. Sunday and was taken to a specific area on the tarmac.
WPTZ anchor Alice Kang said on social media that she was on board the United plane and that she had been stuck on the plane for almost two hours, with several police and emergency crews out on the runway.
In subsequent tweets, Kang reported that the pilot told passengers they were not allowed to take anything from the aircraft's overhead bins, as they were only allowed to remove their phones and IDs as they got off the plane.
According to Kang, there was a note found with a threat, and police K-9's were being used to sweep the plane to look for a possible device.
Pictures from the airport showed a line of ambulances, fire trucks and police cruisers parked out front, and a United airplane at a stop in the middle of the tarmac.
Passengers could be seen deplaning directly onto the tarmac, and at least one police dog walking toward the aircraft.
Other photos showed baggage lined up outside the plane.
Kyle Ambusk told NBC10 Boston that he was out for a walk near the airport Sunday evening when he saw a lot of emergency vehicles on the roadway, blocking off traffic near the airport. According to Ambusk, a driver told him about a potential bomb threat on a plane.
"It was scary," Melissa Jones, a passenger on United Airlines flight 3613, added. "I had to stay calm because of my kids."
There was no immediate word from airport officials on if other flights were delayed as a result of the law enforcement response. Police said they found no credible threat on the aircraft, and they are continuing to investigate.
No one was injured, and no arrests have been made.
"What we have here is a safe group of people. We have a safe plane. We have a safe airport. We have procedures that were followed and we have a successful outcome," Jon Murad, acting Burlington police chief, told WPTZ.