At least seven Los Angeles city firefighters have been injured in a semi-truck explosion and at least two are in critical condition, a department spokesperson said.

The semi-truck was fueled by compressed natural gas and one of the two 100-gallon tanks aboard exploded as firefighters attempted to put out a fire, said Los Angeles Fire Capt. Erik Scott. The other tank is still in place and the gas has not yet been expelled and continues to pose a threat in the immediate area.

A ball of flames reached as high as the top of nearby utility poles burst. “It actually did explode one of the transformers nearby,” Scott said of the ball of flames.

Hazardous material specialists on the scene are monitoring the air in the area and report normal levels at this time. Waves of firefighters will be coming in to fully extinguish the fire that remains as the gas dissipates into the air, Scott explained.

Aerial footage from the scene shows charred firefighter gear and clothing littered on the street near the wreckage of a nearly unrecognizable chassis of a semi-truck. A few cars parked nearby have shattered windows, likely a casualty of the explosion.

The status of the truck driver is unclear and the surrounding circumstances are still under investigation, Scott said.

About a dozen firefighters were on scene when the explosion happened, and two were critically injured. A news conference from the hospital where all seven are being treated is expected later Thursday.

The explosion happened in an industrial part of Wilmington, near the Port of Los Angeles, just west of Long Beach.