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Swedish media say a Stockholm city bus that didn't have any passengers on it caught fire from an explosion in Stockholm, but there were no signs it was a terror attack.

Sweden's Aftonbladet newspaper reported the bus driver was rushed to the hospital but no one else was known to have been injured.

Police spokeswoman Carina Skagerlind told the newspaper an out-of-service city bus powered by natural gas caught fire.

Photos and video footage from the scene show the bus in flames and thick black smoke.

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

Authorities said the bus might have collided with barriers that hang above the tunnel's entrance.