Captain Mike Perry reports the bus caught fire on the southside of the Fort McHenry tunnel around 4:30 Tuesday, inside of northbound Bore 3.
The bus is believed to be a significant distance inside.
Heavy smoke can be seen pouring out of the vent system of the tunnel.
People have been instructed to evacuate their vehicles and are walking toward the roadway at the southside of the tunnel.
So far, two people may be suffering from smoke inhalation.
Fire officials contained the fire around 5 p.m. Tuesday.
Sky Team 11 Capt. Roy Taylor reported a huge backup of traffic during rush hour all the way back to the Beltway. Drivers will have to use Interstate 895 or an alternate route to get home.
Taylor reported that drivers stuck inside the tunnel were evacuated. He said that one person, believed to be the bus driver, suffered minor injuries. No other injuries were reported.