Sources say two people were killed in a vehicle explosion Wednesday afternoon at Rainbow Bridge, a U.S.-Canada border crossing near Niagara Falls, in western New York. Four U.S.-Canada border crossings in the region have been closed as investigators respond to the situation, CBS Buffalo affiliate WIVB reported.

Multiple law enforcement sources confirmed to CBS News two people died inside the vehicle that exploded, and sources said that it appears at this point to have been a deliberate act.

A senior U.S. official confirmed to CBS News a car traveling from Canada at a high rate of speed crashed into a pylon at the bridge crossing. The crash was followed by an explosion, but the U.S. official said it is not yet known why the car exploded. 

A senior administration official describes the incident to CBS News as “more than a freak accident, but we don’t yet have details.”

Mike Guenther, a Canadian resident visiting New York, said he was walking nearby and saw the car speeding towards the border crossing.

“And he was flying, over 100 miles an hour. There was a car in front of him, he swerved out, went in front of the car, hit the fence, went flying up into the air … and we just seen the fireball, and that’s all we could see, there was just smoke everywhere,” he said.

Guenther said the car was heading from the U.S. toward Canada, which differed from other preliminary accounts.

The City of Niagara Falls said the incident involved a vehicle attempting to enter the U.S., WIVB reported.

The FBI Buffalo Field office posted a statement on social media saying it was “investigating a vehicle explosion at Rainbow Bridge, a border crossing between the U.S. and Canada in Niagara Falls.” It added that the situation is very fluid and “that’s all we can say at this time.”

New York Gov. Kathy Hochul said in a statement that she has been briefed on the situation. “At my direction, the New York State Police is actively working with the FBI Joint Terrorism Task Force to monitor all points of entry to New York,” she said, adding that she is on her way to Buffalo to “meet with law enforcement and emergency responders.”

U.S. travelers during one of the busiest travel periods of the year can expect to see an increase in law enforcement and canines patrolling the transportation system, the Transportation Security Administration said in a statement. The TSA said it is “operating at a heightened level of security as a result of world events and the current threat environment.”

The Rainbow Bridge is a popular crossing for tourists, connecting Niagara Falls, New York, with Niagara Falls, Ontario, Canada. The other border crossings in the region are the Peace Bridge, the Whirlpool Rapids Bridge and the Lewiston-Queenston Bridge.

This is a breaking news story. Please check back for updates. Pat Milton, Robert Legare and Andres Triay contributed reporting.