A vehicle exploded at a checkpoint on the American side of a US-Canada bridge in Niagara Falls, leaving two people dead and prompting the closing of four border crossings in the area, authorities said.
There was no immediate information on the cause of the explosion, but it 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,” Mr 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 said.
The US FBI’s field office in Buffalo and other agencies were investigating the blast.
New York governor Kathy Hochul was traveling to Buffalo from the state capital, Albany.
The explosion happened on the US side of the Rainbow Bridge, which connects the two countries across the Niagara River.
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,” Mr 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 away from the popular tourist destination.
Ms 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.
Mr Trudeau said “additional measures” were being contemplated and activated at border crossings across Canada.
In Toronto, about 100 miles away, police said they were increasing patrols as a precaution.
New York City police were monitoring the news from Buffalo but already had boosted security at various spots because of the Macy’s Thanksgiving Day Parade on Thursday.
The Niagara Falls Bridge Commission reported that all four of its crossings, the others are Lewiston, Whirlpool and Peace Bridge, were closed.
Sanchit Chatha, his wife Reyshu and their 13-year-old daughter, Trisha, had stopped in Niagara Falls for lunch en route home to Toronto from Buffalo when they started getting news notifications about the explosion.
Worried friends called, knowing the family was in the area.
Trisha was concerned at seeing the bridges to Canada shut down, her mother said.
“She has a maths test tomorrow,” the mother explained as the family waited to find out when the crossings would open.
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.
The bridge, constructed in 1941, is just over 1,440 feet (439 meters) long and has a main span constructed of steel, according to the data.
