(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 connecting Canada to New York State went through the initial checkpoint passports and licenses are examined. The car was 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.

“This is obviously a very serious situation,“ Canadian Public Safety Minister Dominic LeBlanc told reporters Wednesday.

“We’re taking this circumstance very seriously but to speculate on the origin of this particular circumstance – the reasons why this may have happened – until we have more accurate information is simply not responsible.”

Four photos circulating on social media show the remains of a vehicle strewn about the US-side of the border crossing. CNN has geolocated and confirmed the authenticity of the four photos.

Thick smoke and fire can be seen in the photos. One image shows a border guard shack with significant damage and charred debris next to it.

The FBI, New York State Police and US 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 are 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.

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 US 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 US 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 US near Niagara Falls, according to US Customs and Border Protection.

This is a developing story and will be updated.

CNN’s Laura Dolan contributed to this report.

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