Rainbow Bridge between US, Canada closed following vehicle ...

The Rainbow Bridge connecting the U.S. and Canada at Niagara Falls, New York, has been closed after a vehicle explosion, according to the FBI and local authorities.

There are believed to be fatalities from the incident, multiple officials confirmed to ABC News.

The Niagara Falls mayor's office said the incident involved a vehicle crossing into the U.S. from Canada.

Surveillance video showed the vehicle stopped at an initial security checkpoint, according to sources briefed on the investigation. The vehicle was then directed to a secondary security checkpoint, and at that point the vehicle sped up and crashed into the secondary screening location, sources said.

What triggered the initial security stop was not immediately clear. It is also not immediately clear what caused the explosion.

Investigators have found some sort of suitcase or briefcase on scene, sources told ABC News. They are treating it as a possible explosive device as a precaution and the bomb squad is handling the package.

Ron Rienas, GM of the Buffalo and Fort Erie Public Bridge Authority, told ABC News all four Canada-U.S. bridges over the Niagara River have been closed out of an abundance of caution while the Rainbow Bridge investigation continues.

New York Gov. Kathy Hochul said, "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. I am traveling to Buffalo to meet with law enforcement and emergency responders and will update New Yorkers when more information becomes available."

The Buffalo Niagara International Airport said it'll be increasing security, with car checks and additional screenings for travelers.

The White House is also monitoring the situation, a White House official said.

"We are taking this extraordinarily seriously," Canadian Prime Minister Justin Trudeau said. "We are following up to try and get as many answers as rapidly as possible. … Additional measures are being contemplated and activated at all border crossings across the country."

Ontario Premier Doug Ford said, "Our provincial law enforcement is actively engaged in assessing the situation. They are working with local law enforcement and are providing support."

This is a developing story. Please check back for updates.