A school bus caught fire in the Fort McHenry Tunnel this afternoon, closing six of its eight traffic lanes and causing significant traffic backups, but police said the blaze was extinguished without injuries.

Sgt. Jonathan Green, a spokesman for the Maryland Transportation Authority Police, said a school bus occupied by a driver and an assistant caught fire about 4:30 p.m. in the left northbound bore of the four-bore tunnel.

Green said the two occupants escaped the bus without injury. He said there were no children on the bus.

The two northbound bores and the two lanes of the inside southbound bore were closed, Green said. He said the two lanes of the bore where the fire took place will reopen after the vehicle is removed and the tunnel is inspected for structural damage. The other lanes reopened by 6:30 p.m.

Green said the vehicle became disabled in the southern half of the tunnel. The authority detoured drivers onto the Beltway (the Key Bridge) and Interstate 895 (Harbor Tunnel). State Highway Administration video cameras showed a backup that appeared to stretch for several miles on northbound Interstate 95 approaching the tunnel.