All lanes reopened in Fort McHenry Tunnel on Wednesday morning after a school bus caught fire Tuesday afternoon, closing six of its eight traffic lanes and causing significant traffic backups, police said.

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

Two occupants escaped the bus without injury, Green said. No children were on the bus, he said. The flames were caused by some mechanical failure, Green said.

The two northbound bores and the two lanes of the inside southbound bore were closed, Green said, but were reopened by 4:45 a.m. Wednesday.

It initially was unclear when the lanes would reopen because of the extensive damage inside, including smoke-damaged light fixtures, melted electrical wiring, and loose exterior tiles inside the tunnel; however, the engineers say there was no damage to the tunnel's structure. Debris and residue remained on the road Tuesday night, Green said.

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.

michael.dresser@baltsun.com