Niagara Falls, New York - US terrorism investigators were deployed Wednesday after a car erupted into a fireball at a US-Canada checkpoint, triggering border closures on one of the busiest travel days of the year.

Two people who were both in the car died in the blast, although their identities have not yet been made public.

The vehicle, which was driving from the US into Canada, exploded at the major Rainbow Bridge crossing near Niagara Falls, local and state authorities said, which marks the boarded from New York State with Canada.

authorities on both sides of the border reacted urgently. The White House said that President Joe Biden had been briefed. Canadian Prime Minister Justin Trudeau told parliament "this is obviously a very serious situation" and announced the closure of four crossings.

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New York Governor Kathy Hochul said state police and the FBI Joint Terrorism Task Force were monitoring all points of entry into the state, and later confirmed at a press conference "there is no indication of a terrorist attack."

US media cited law enforcement sources as saying there were two people in the car, which had passed through one border checkpoint, where it was selected for a secondary check. The car then sped up, crashed into a barrier and exploded, the sources said, stressing it was unclear if the blast was the result of a device.

Mike Guenther, a Canadian visiting the United States, described a dramatic, high-speed crash at the checkpoint. "We’ve seen this car coming down towards the border and he was flying – over 100 miles an hour," he told CBS News.

The car then swerved and "hit the fence, went flying up into the air," he said. "He went up into the air and we just seen the fireball and that's all we could see. It was just covered in smoke everywhere."

The incident came on the eve of the Thanksgiving holiday, one of the busiest days for travel on the US holiday calendar, when millions of Americans take to the roads and skies.

All four bridges between the countries were immediately closed after the incident, and at least two have since reopened. Buffalo Airport was initially closed to departing and arriving international flights.

Rainbow Bridge – among the busiest crossings between Canada and the US – has 16 vehicle lanes and is normally open around the clock, according to US Customs and Border Protection.

Canada's Public Safety Minister Dominic LeBlanc said "any time... [infrastructure like] a border crossing sees this kind of violent circumstance, it's a source of concern for" Washington and Ottawa.

Hochul announced that she was traveling to Buffalo to meet with law enforcement and emergency responders and would update New Yorkers when more information became available.

Trudeau's office said he had been briefed and was in contact with US law enforcement. Multiple Canadian agencies were supporting the investigation, it added.

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Multiple witnesses reported hearing a loud explosion and seeing a large cloud of smoke near the inspection station.

Ivan Vitalii, a visitor from Ukraine, told the Niagara Gazette he and a friend were at a nearby convenience store on the US side when they saw a car exit a parking lot and travel toward the bridge.

"We heard something smash," he said. "We saw fire and big, black smoke."

Dramatic images posted to social media, which AFP has not verified, showed thick plumes of black smoke rising from a conflagration at the crossing.