DENVER (AP) — Unstable rubble hampered investigators’ efforts Friday to determine what caused a thunderous explosion that killed one person, injured at least seven and leveled a downtown restaurant and a store in the southern Colorado city of Pueblo.

“We still don’t know exactly what happened,” Pueblo Fire Department spokesman Gary Micheli said.

The mounds of rubble kept federal and state officials from getting a close look at the destruction to find possible causes of Thursday’s blast. Investigators wanted to inspect a natural gas line and a gas meter at the site of the restaurant, said Woody Percival, another fire department spokesman.

City arson investigators and agents from the Colorado Bureau of Investigation were at the scene. Agents from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were expected to arrive today.

“That’s not to say whether we suspect anything at this point in time,” said Police Chief John Ercul. “It may be an accident.”

Micheli said the investigation could take up to a couple months.

A gas leak was found in a street next to the destroyed Branch Inn restaurant and A Classic Boutique store. But it wasn’t clear whether the pipe was ruptured by the explosion or contributed to it, Percival said. The break was in a main gas line about 20 feet from the point where a feeder line runs to the building that exploded. At least one witness reported smelling gas after the blast.

Thursday’s explosion at 2:10 p.m. MDT spewed glass, brick and other debris more than 100 feet and damaged surrounding buildings and cars. A door hinge was imbedded in the fender of one SUV.

Ashley R. Johnson, 22, who had been in the boutique, died at Parkview Medical Center, Pueblo County Coroner James Kramer told The Pueblo Chieftain.

Eugene Roney, 74, who had been in the restaurant, was in critical condition at Parkview, police said. Also hospitalized were Frank Vigil, 41, co-owner of the Branch Inn; Kevin Trujillo, 40; Jennifer Flower, 23; and Nick Stabile, 18, the Chieftain reported. At least one other person was taken by private vehicle to a medical center.

Flower was flown to a Denver hospital for treatment. Trujillo and Stabile were treated and released, and Vigil was listed in fair condition.

One survivor was found in the rubble more than four hours after the blast, but it wasn’t immediately known which one.

Ercul said the restaurant was open at the time of the explosion.

Christine Guerin, 35, said she was stopped at a stop sign when she heard the explosion and saw glass flying toward her.

“You couldn’t even see the building. It was just smoke, black smoke,” Guerin said. She added that she heard a hissing noise and smelled gas when she got out of her car.

Pueblo, a city of about 106,000.

, is 100 miles south of Denver.