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DENVER — Nine people were injured, one critically, by an apparent natural gas explosion on Tuesday afternoon that firefighters deemed “suspicious,” the Denver Fire Department said.

The blast collapsed the roof and tore the walls off of a four-plex residential building at the intersection of West Fourth Avenue and Santa Fe Drive.

Fire investigators said they have not ruled out arson, and the Denver Police Department crime lab was on the scene.

Fire department spokesman Capt. Greg Pixley said seven people were treated for minor injuries and released after a portion of a brick rowhouse was leveled in the blast.

A man was critically injured and a woman who was pulled from the rubble is in stable condition.

Denver Fire said rescuers were confident there were no more victims in the rubble.

Pixley said some of the injured were burned, and the fire department is working with Xcel Energy to determine what happened.

A man was driving his car in front of the building when it blew up.

One witness said the structure is an apartment building with five or six residents.

A man living four blocks away said when he heard the blast, he thought the roof of his home was caving in.

Alan Rodriguez said when he went outside to see what happened, he saw a “mushroom cloud” of smoke.

Richard Trujillo lives nearby and said he was walking to a store when he smelled gas before the building “just blew up” and bricks were flying into the street.

He said that just after the explosion, a man was getting out of a car when a person “dragged him away and took a baby out of the car.”

Most streets in the area were reopened Tuesday night.