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A bus exploded after crashing into a barrier near the entrance of a tunnel in Stockholm, Sweden, on Sunday, police said.

The city’s police department tweeted that the driver of the bus was rushed to the hospital with burns. No passengers were on the bus.

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The department said the incident was being investigated as a traffic accident.

Photos from the scene posted on social media showed the bus consumed in flames and thick black smoke rising from the wreckage.

The Stockholm police told the Swedish newspaper Aftonbladet that the bus ran on natural gas.

Stockholm fire and rescue officials were called to the Klara Tunnel in central Stockholm at about 11:25 a.m. local time.

A witness, Björn Wallentin, told Aftonbladet: “I heard a big explosion and then several smaller ones. It sounded like the scaffolding of a building falling down. I thought it was a terror attack.”

The newspaper said the out-of-service bus was on its way to a bus depot in the northern part of the city where the driver's bus route was to start.