GAITHERSBURG, Md. (AP) — Twelve people were injured, two of them critically, in an explosion and fire at a Maryland apartment building Wednesday and the residents of one unit have not been accounted for, a fire official said.

Fire crews responded to the garden apartment complex on Quince Orchard Boulevard in Gaithersburg around 8:40 a.m. for a building explosion and fire with a partial collapse, Montgomery County Fire Chief Scott Goldstein said at a news conference. Maintenance staffers used a ladder to rescue two people from the building, including one person on a rear balcony, Goldstein said.

Two people were treated on the scene and another 10 were taken to hospitals, including two adults in critical condition, Goldstein said. The remaining eight, including four children, were taken to hospitals with mild to moderate injuries.

Most of the occupants of the two directly affected buildings have been contacted and are accounted for, but officials have not been able to reach the residents of one unit, Goldstein said.

Other buildings in the surrounding areas have been checked and there are no concerns about gas in those buildings for residents who are able to reoccupy their homes Wednesday night, he said.

By mid-morning, firefighters had knocked down the blaze and there was a gas-fed fire in the building until the gas service was shut off, he said.

People reported to investigators on the scene that there had been a smell of gas Wednesday morning, but Goldstein said there were no 911 calls reporting a gas leak before the explosion. The last gas-related call in one of the affected buildings was in late September, he said.