SILVER SPRING, Md. (AP) — Investigators have found a cut gas pipe in the basement of a Maryland four-story apartment building destroyed by an explosion and fire, but investigators are working multiple theories on the cause, a fire official said Friday.

It is possible a maintence worker who was working on plumbing at the Friendly Garden Apartments in Silver Spring at the time of Thursday’s explosion cut the gas pipe, Montgomery County Fire Chief Scott Goldstein said at a briefing. It’s too early to say whether it was cut accidentally, he said. Investigators don’t have an ignition source, he said. The worker, who was injured in the blast, has been able to talk to investigators, he said.

All known residents of the building have been accounted for a day after the fire, Goldstein said. Some residents got out with help from fellow residents or on their own, one resident jumped from a second-story window and at least one was helped out by fire personnel, Goldstein said.

Of the 12 adults and two children who went to hospitals, three remain in serious condition, he said. He added that seven have already been discharged.

While there are no reports of missing residents, Goldstein said human remains dogs alerted in the front left corner of the building and investigators are operating under the belief that there’s a person there, removing debris by hand.

“We want to rule out that there was somebody who was in front of the building at the time of the blast,” he said.

More than 200 residents were displaced overnight from the six buildings in the complex just north of Washington, D.C. , and three of those buildings were deemed unsafe to occupy, he said. Power was restored to the other three buildings and officials hope residents can return Friday night, he said.

A nearby resident’s outdoor security camera recorded the explosion, which destroyed an end unit of one building. Other video from the scene showed flames shooting from the roof and debris scattered over the ground.