A vehicle exploded at a checkpoint on the American side of a US-Canada bridge in Niagara Falls Wednesday, leaving two people dead and prompting the closing of four border crossings in the area. There was no immediate information the cause of the explosion, but it raised concerns on both sides of the border.

Details of what led to the crash remain scarce, with security camera video released by the US government showing the car speeding through an intersection, hitting a low median and flying through the air into a line of vehicle checkpoint booths on the American side of the crossing.

However, the FBI said the crash showed no sign of terrorist involvement and there were no explosive materials at the scene, and the case has been turned over to the Niagara Falls Police Department as a traffic investigation.

Federal Public Safety Minister Dominic LeBlanc stressed it is “simply not responsible” to speculate on what caused the incident without more accurate information.

Prime Minister Justin Trudeau said officials were “taking this extraordinarily seriously,” and the White House said President Joe Biden was “closely following developments.”

The “fireball” crash occurred on one of the US’s busiest travel days of the year and prompted deployment of the FBI Joint Terrorism Task Force, as well as state police. Law enforcement officials are reported as saying that the car had passed one border checkpoint but took off when it was selected for a secondary check before crashing into a barrier.

The two deceased people were in the vehicle, a law enforcement official said.

“This is obviously a very serious situation in Niagara Falls,” Trudeau said in Parliament before he excused himself from Question Period in the House of Commons to be briefed further.

Three other bridges between western New York and Ontario were quickly closed as a precaution, and the Buffalo-Niagara International Airport began security checks on all cars and told passengers to expect additional screenings.

Trudeau said that “additional measures” were being contemplated and activated at border crossings across Canada.

Photos and video taken by bystanders and posted on social media showed thick smoke, flames on the pavement and a security booth that had been singed by flames.

Videos showed that the fire was in a US Customs and Border Protection area just east of the main vehicle checkpoint.

Speaking to WGRZ-TV, Mike Guenther said he saw a vehicle speeding toward the crossing from the US side of the border when it swerved to avoid another car, crashed into a fence, and exploded.

“All of a sudden he went up in the air and then it was a ball of fire like 30 or 40 feet high,” Guenther told the station. “I never saw anything like it.”

From inside Niagara Falls State Park, Melissa Raffalow said she saw “a huge plume of black smoke” rise up over the border crossing, roughly 45 metre away from the popular tourist destination.

The Niagara Falls Bridge Commission reported that all four of its crossings – the others are Lewiston, Whirlpool, and Peace Bridge – were closed.

About 6,000 vehicles cross the Rainbow Bridge each day, according to the US Federal Highway Administration’s National Bridge Inventory. About 5% is truck traffic, according to the federal data.