This post has been updated.

The 3rd Street Tunnel was temporarily closed in both directions after a minor explosion Monday morning, D.C. fire officials said.

A tow truck was transporting an unoccupied school bus at 10:42 a.m. when “there was some kind of explosion in the bus,” said D.C. fire spokesman Lon Walls.

The nature of the explosion remains under investigation, Walls said. The tow truck driver and other motorists evacuated their vehicles, and no injuries have been reported.

John Lisle, spokesman for the D.C. Department of Transportation, said it appears that as the bus was being towed, it may have struck some of the lights on the ceiling of the tunnel, sparking a fire. The accident happened on the northbound side of the tunnel.

Crews from the D.C. Department of Transportation remained on the scene for several hours inspecting the tunnel. DDOT spokesman John Lisle said all lanes were to reopen about 3 p.m., he said.