A car erupted into a fireball at a US-Canada checkpoint near Niagara Falls on Wednesday, killing the two occupants, triggering border closures and sparking a massive security alert on the eve of a major holiday.

The regional FBI field office concluded the blast, which occurred around 400 miles (640 kilometers) northwest of New York City, was not linked to terrorism.

Witnesses described seeing a car traveling at high speed before it crashed into a checkpoint barrier and exploded into flames.

The blast happened at the major Rainbow Bridge crossing near Niagara Falls with nothing left of the vehicle which was incinerated except for the engine, according to New York Governor Kathy Hochul.

Confirming the two fatalities, she suggested the vehicle may have been from western New York state. The victims' identities have not yet been made public.

Authorities on both sides of the border activated emergency responses and images showed access roads to the crossing swarming with emergency service workers and vehicles.

The White House said President Joe Biden had been briefed and was closely following developments.

Canadian Prime Minister Justin Trudeau told parliament "this is obviously a very serious situation."

'He was flying'

"We've seen this car coming down towards the border and he was flying — over 100 miles an hour," Mike Guenther, a Canadian visiting the United States, 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 US travel days when millions of Americans take to the roads and skies.

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

Hochul said that debris was spread across as many as 14 of the lanes following the incident.

Three other nearby border crossings had been closed in the aftermath of the incident, but had since reopened, US Customs and Border Protection said.

On the US side, Niagara Falls State Park, which attracts millions of visitors every year, was closed until further notice according to the park service.

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" US and Canadian authorities.

Trudeau's office said he had been briefed and was in contact with US law enforcement.

The regional FBI field office located in the city of Buffalo, New York said a search had revealed "no explosive materials, and no terrorism nexus was identified."

It added that the matter had been turned over to local police as a traffic investigation. 

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

Flames, thick smoke

Canadian Chorkor Millionaire was accompanying friends back across the border into his home country when they heard a loud boom.

There was a vehicle "caught up in flames, thick smoke all of the sudden," he told AFP, adding: "Everybody was terrified."

New York City Mayor Eric Adams said that officers of the city's police department, the country's largest, had deployed to the border crossing to assist the investigation.

Dramatic videos showed thick plumes of black smoke rising from a conflagration at the crossing.