An apartment exploded in the 4300 block of South Quebec Street Thursday morning.

When crews arrived, they found heavy smoke and fire pouring out of the building.

Two units in the building were heavily damaged and there was extensive damage to the roof.

Witnesses saw the massive flames and explosion.

“I saw it [the flames] jump from the first part to the second part of the building and things just started exploding, like aerosol cans or something,” Steven Acton, who has been living at the complex for a week, said.

“I just kept hearing explosions man, didn’t know what the heck it was, sounded like gunshots,” Acton said.

Crews are still working to make sure all hot spots are put out.

Denver Police and Denver Fire are working to determine the cause of the fire.

According to Luis Cedillo with Denver Fire, the possibility that the fire was caused by a hash oil extraction explosion is being investigated.

“We don’t know who was inside or who was participating, it could have been a solo operation, we don’t know that," Cedillo said.

Several people were injured.

“At the moment we have two injuries," Cedillo said. "One civilian and one firefighter were transported. We don’t know the extent of the injuries for the civilian. At the moment, the firefighter’s injuries are minor.”

The firefighter is expected to be released from the hospital this afternoon.

The civilian who was injured is in custody. It's unclear if he is in the hospital, or in jail.

At this point, that's the only person in custody.