NEW YORK (CBSNewYork) — A Germany-bound plane from Houston was forced to land at John F. Kennedy International Airport due to a threat made against the aircraft, officials said Monday night.

As CBS2’s Valerie Castro reported, Lufthansa flight 411, which was headed for Frankfurt, made an emergency landing late Monday after a bomb threat was made over the phone Monday evening, the Port Authority of New York and New Jersey said.

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The threat was called into Lufthansa’s corporate offices and when the pilot was informed, he chose to divert to JFK, Port Authority sources told CBS2.

Authorities were treating it as a viable threat and the NYPD Counterterrorism Unit is responding along with the Port Authority Police Department, sources told CBS2. But there was no indication of trouble beyond the threat by phone.

Police said the aircraft was moved to a secure area away from the airport and passengers on the plane were evacuated late Monday. A massive emergency response surrounded the plane once it was on the ground.

A Port Authority source said 530 passengers had been on board. The passengers were put onto buses shuttled to an unknown location.

One passenger spoke to CBS2 by phone and described the situation.

“Everybody’s still pretty calm. The situation is obviously, you know, a bit tense, but so far, things are pretty calm,” the passenger said.

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But the passenger said no such thing had ever happened when he was flying.

“It’s definitely weird. This is – I’ve been flying pretty consistently for the past few years and this never happened before,” the passenger said. “So I guess, when I guess, one of the pilots announced a bomb threat, to me, it felt a bit surreal. It took me a little bit of time for that to actually sink in.”

The Lufthansa flight took off from Houston earlier Monday evening, and passed over Louisiana, Mississippi, Tennessee, Kentucky, West Virginia, Virginia, Maryland, Delaware and New Jersey. Once the plane was over the area of Southampton, Long Island, the flight path took a turn and headed back toward JFK.

The aircraft was being searched late Monday, Lufthansa said. A K9 team had to go through the aircraft, and all luggage and cargo was being checked, Port Authority police said.

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According to the Federal Aviation Administration, delays at JFK were estimated at between 15 and 29 minutes in length late Monday night.

It was still unclear late Monday what the threat said specifically, and whether or not authorities found any kind of device on the plane.

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The Port Authority said similar threats were made over the weekend against other flights, including one that was destined for JFK.