NIAGARA FALLS, New York (AP) — New York Governor Kathy Hochul said there was “no indication of a terrorist attack” after a vehicle speeding toward a US-Canada bridge from the American side crashed and exploded at a checkpoint in Niagara Falls on Wednesday, killing two people and prompting the closing of border crossings in the area.

Much remained unclear about the crash, which investigators were working to understand. Hochul said it wasn’t clear whether the driver was intentionally heading for the Rainbow Bridge across the Niagara River when the vehicle hit a median and flew into the air.

“Based on what we know at this moment,” she said, “there is no sign of terrorist activity in this crash.”

It prompted the closing of four border crossings and raised concerns on both sides of the border. The White House said President Joe Biden was “closely following developments,” and Canadian Prime Minister Justin Trudeau said officials were “taking this extraordinarily seriously.”

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

The two deceased people were in the vehicle, a law enforcement official told The Associated Press. The official was not authorized to discuss details of the investigation publicly and spoke on condition of anonymity.

Aaron Ferguson, a spokesperson for the city of Niagara Falls, New York, said the vehicle was travelling at high speed from that city and crashed into the border station.

Jim Diodati, the mayor of Niagara Falls, Ontario, told The Canadian Broadcasting Corporation that what happened appeared to be “an isolated incident.”

The US FBI’s field office in Buffalo and other agencies were investigating the blast.

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. Videos showed that the fire was in a US Customs and Border Protection area just east of the main vehicle checkpoint. The agency had no immediate comment.

Speaking to WGRZ-TV, witness Mike Guenther said he saw a vehicle speeding toward the crossing from the US side 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.”

The Niagara Falls Bridge Commission called the incident “a serious car crash.”

From inside Niagara Falls State Park, Melissa Raffalow said she saw “a huge plume of black smoke” rise up over the border crossing, roughly 50 yards (45 metres) away from the popular tourist destination. Raffalow told AP in a message that police arrived soon after, urging visitors to disperse as they began cordoning off the street.

Raghu Bhattarai said by phone that he was inside his restaurant, the Niagara Tandoori Hut, near the bridge when he heard a sound he described as a “boom.” A few minutes later, he saw black smoke rising.

The bridge and three others 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.

The safety measures tied up traffic at the airport and elsewhere on one of the busiest US travel days of the year, ahead of the American Thanksgiving holiday.

Trudeau said “additional measures” were being contemplated and activated at border crossings across Canada. In Toronto, about 100 miles (about 160 kilometres) away, police said they were increasing patrols as a precaution.