A Metrobus that caught fire Thursday evening caused a plume of heavy black smoke to cover part of Downtown D.C.

The bus caught fire in the 9th Street Tunnel around 7 p.m. The bus driver was not injured and no one else was riding the bus when the fire started, Metro said.

The cause of the fire is still unknown.

At a news conference Thursday night, Metro General Manager and CEO Randy Clarke said because the bus was a hybrid diesel vehicle, out of an abundance of caution, all 32 of the buses in Metro’s fleet are being removed from service and will be inspected.

A spokesperson with D.C. Fire and EMS confirmed with WTOP that the tunnel would be closing as personnel worked to put out the fire. The tunnel and 9th Street Expressway between Constitution Avenue NW and Interstate 395 are closed.

“We are working closely with the manufacturer and our bus team is developing plans to mitigate any impacts to bus service as a result of us taking this proactive measure,” WMATA said in a post on social media.

WTOP Traffic reporter Ian Crawford said firefighters could be seen blocking traffic and entering the tunnel with water hoses.

D.C. Fire Chief John Donnelly said servicing the fire “was a tough operation” because firefighters had to pull about “900 feet of hose and carry it in there.”

“It’s an operation we train for. It’s one we don’t do very often, though,” he said. “It’s a hard tunnel to come into because it’s one way.”

Traffic cameras showed the plume of smoke pouring out of the tunnel and hanging over D.C. for more than an hour before dissipating around 8 p.m.

It is unclear if the tunnel will be open to vehicle traffic on Friday.

This is a developing story. Stay with WTOP for the latest.