(CNN) — The two occupants of a car that exploded Wednesday at the US-Canada Rainbow Bridge border crossing are dead, a law enforcement source told CNN.

The incident occurred shortly before noon when a car coming from the Canadian side of the border on the bridge that connects Canada to New York State went through the initial checkpoint where they examine passports and licenses and then was directed to another area for a secondary search, according to multiple law enforcement sources.

As the car was heading towards the secondary search area, an explosion occurred, the sources said. Authorities do not know what caused the explosion.

The FBI, New York State Police and U.S. Customs and Border Protection are on the scene investigating. Federal authorities are trying to retrieve video from a number of different angles because the statements from witnesses vary, the sources said.

Sources said there were no serious injuries except to those who were in the car.

Federal authorities in Buffalo, New York, said the “situation is very fluid.”

The FBI is coordinating with local, state and federal law enforcement in the ongoing investigation, it said on X, formerly known as Twitter.

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

Earlier, the Niagara International Transportation Technology Coalition reported the bridge was closed on both sides due to a crash.

A vehicle was trying to enter the U.S. side of the border, according to a spokesperson for the Niagara Falls mayor’s office.

“We don’t really know too much – just that there was a vehicle trying to come into the U.S., and they shut down both sides of the bridge,” Aaron Ferguson, public information officer for the city of Niagara Falls, told CNN.

The bridge was reported closed at 11:51 a.m., according to the technology coalition. Camera footage from the coalition shows haze surrounding the bridge.

There are four ports of entry between Canada and the U.S. near Niagara Falls, according to US Customs and Border Protection.

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