DENVER (Reuters) - Nine people were injured, one critically, on Tuesday when a natural gas explosion leveled a Denver apartment house, a fire department spokesman said.

People watch as emergency personnel work at a site where a natural gas explosion leveled an apartment building in Denver, Colorado, August 14, 2018, in this picture obtained from social media. Chelsea Sheehan/via REUTERS

First responders rescued two victims trapped inside the building, described as a “fourplex,” in Denver’s historic Baker neighborhood, said Denver Fire Captain Greg Pixley.

No one else was believed trapped in the rubble, alive or dead, Pixley said, but firefighters were conducting a thorough search to make sure.

“What we are using now is the cadaver dog ... to help us assure that we haven’t missed anything,” he said. “Our goal is not to find a victim, but to rule out that a victim is not there.”

Pixley said that arson investigators and a crime scene unit was on the scene as part of the investigation and that crews for Xcel Energy Inc XEL.O, the Denver utility that supplied the building, had shut off all gas lines into the area.

Xcel Energy spokeswoman Michelle Aguayo said by phone the utility was working with the fire department to determine the cause of the blast.

Two people hurt in the explosion were transported to Denver Health Medical Center, said hospital spokesman Simon Crittle. One had severe traumatic injuries while the other’s injuries were not as severe, he said.

Seven other people who suffered minor injuries were treated at the scene, the Denver Fire Department said on Twitter.

A man who lives in the area told Denver television station FOX31 he smelled gas as he was walking to the store, before an explosion sent bricks flying into the street.

The fire department posted pictures of the apartment building on Twitter showing its roof collapsed in the explosion, which propelled wooden beams and other rubble cascading across the sidewalk.