The Rainbow Bridge that connects the United States and Canada at Niagara Falls is closed after an explosion occurred on the U.S. side of the border crossing.

Initial reports have stated that the explosion involved a vehicle that was crossing the bridge into the U.S. Niagara Falls Mayor Robert Restaino said the vehicle was crossing into the U.S. from Canada via ABC News.

The FBI in Buffalo, New York, said in a statement that agents are investigating a “vehicle explosion” and working with local, state, and federal law enforcement.

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“As this situation is very fluid, that’s all we can say at this time,” the FBI said.

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

In response to the vehicle explosion, the Niagara Falls International Airport is increasing security at the airport and rail system. The Buffalo Niagara International Airport is open but K-9 units will be checking vehicles coming through the airport.

Gov. Kathy Hochul (D-NY) said she is monitoring the situation closely.

Photos shared with local outlet WGRZ-TV by a witness show the remains of what appears to be a vehicle, smoke, and flames.

This is a developing story and will be updated.