Burlington, Vermont - A passenger aboard a United Airlines flight left a threatening message on the plane and set off an emergency response that lasted hours.

On Sunday, United Airlines flight 3613 took off from Newark Liberty International Airport in New Jersey.

According to NBC5, about 20 minutes before the plane was set to land at Burlington International Airport in Vermont, the pilot was notified that a threatening message was found in the bathroom.

The pilot notified Air Traffic Control, which launched an emergency response as the plane landed at its destination around 4:30 PM local time.

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The plane was directed to an isolated part of the tarmac where the Vermont State Police Bomb Squad and other law enforcement agencies were waiting.

Passengers were told very little as officers brought in K-9 units to do a thorough sweep of the plane.

"They sniffed up and down the plane, and then they sniffed each passenger," Vermont Sen. Phil Baruth, who was among the 65 passengers, told The Daily Mail.

After two hours, everyone was instructed to exit the plane, but were restricted from bringing any personal items with them as authorities continued their search.

By about 7 PM, authorities determined there was no threat on the plane, and allowed the passengers to retrieve their belongings. Law enforcement still hasn't revealed the true nature of the note or what it said.

Burlington International Airport, which closed during the emergency response, reopened around 7:30 PM.

"It feels unfair on one hand that someone left a note and ruined everyone's day. Obviously someone a little deranged," Senator Baruth said. "On the other hand, authorities here handled everything perfectly and at no time did I feel unsafe."

The investigation is ongoing.