Local and national authorities in the United States and Canada are investigating a vehicle explosion on the Rainbow Bridge.

The cause of the incident was not immediately clear, but New York Governor Kathy Hochul said state police and the FBI’s Joint Terrorism Task Force were monitoring all points of entry into the state.

“I am traveling to Buffalo to meet with law enforcement and emergency response teams, and will update New Yorkers when we have more information,” Hochul wrote on social media.

The FBI’s Buffalo city office said in a statement that it was “investigating a vehicle explosion” on the bridge, adding that the situation was “very fluid, which is all we can say at this time.”

Canadian Public Affairs Minister Dominic Leblanc told reporters that the Canadian government is taking the situation “extremely seriously.”

“We are in close contact with the US authorities,” he added.

Two people in the vehicle were killed. An American officer was also injured, but with non-life-threatening injuries.

Authorities have not yet ruled out the possibility that it was an attack.

Some witnesses said they heard a deafening noise and saw a long column of smoke near the border control.

Ivan Vitalii, a Ukrainian visitor, told the Niagara Gazette that he and a friend were in a nearby store on the U.S. side when they saw a car leaving a parking lot and heading toward the bridge. “We heard something crashing,” he said. “We saw fire and big black smoke.”

Ron Rienas of the Buffalo and Fort Erie Public Bridge Authority told ABC News that all four bridges connecting the two countries over the Niagara River have been closed. (JO)