Gaithersburg, MD — A condo explosion in Gaithersburg, Maryland, left a dozen people injured, two of them critically, kicking off a massive search-and-rescue operation Wednesday, the Montgomery County fire chief said.

The “significant” blast resulted in a structural collapse, Montgomery County Fire Chief Scott Goldstein said. Light smoke continued to rise from the structure around midday.

Authorities are “still working on the basis that there are unaccounted-for personnel,” Goldstein said. “We are diligently working to identify all the occupants.”

At least 100 firefighters responded to the scene after receiving reports of an explosion and fire around 8:40 a.m., fire department spokesperson Pete Piringer said. The complex is composed of three-story buildings housing about 150 units, according to rental sites. Brown Station Elementary, less than a quarter mile west of the complex, was sheltering in place, CNN affiliate WUSA reported. The students and faculty were safe, Piringer told the station.

Washington Gas “personnel are responding to assist the fire department on the scene. … We are supporting the active response to the incident and do not have further details to share at this time,” its statement said.

Crews battle a blaze at a partially collapsed building in Montgomery County, Maryland, on Wednesday.

Ten people were taken to hospitals, including two adults in critical condition. Four adults and four children were transferred with mild to moderate injuries. Two individuals were treated on scene and declined to go to the hospital, Goldstein said.

The fire impacted two condo buildings with six units. Two other buildings were evacuated. Authorities are homing in on nine apartments, trying to account for each unit’s residents, Goldstein said.

Residents told authorities they smelled gas Wednesday morning. Maintenance staff on site used a ladder to rescue a resident from a rear balcony and assisted a resident at the back of the building, the fire chief says.

Tyrell Singleton, who lives nearby and heard and felt a loud explosion before going to check on his brother-in-law’s family, who lives in the neighborhood, told CNN affiliate WJLA that he initially thought it might be an earthquake.

“It was loud,” Singleton said, leaving him to believe something had happened closer to his own home. Once they realized the commotion was coming from about 500 feet away, he and his stepfather ran over to inspect, he said.

“The whole front of the interior of the building was gone. We seen debris, family members, things all on the grounds, fire and just thick smoke,” he told the station.

The local 911 center received “no calls prior to the explosion for a gas leak or a gas smell,” Goldstein said, adding authorities were looking into whether there were reports of a gas odor last week.

The fire remained active late Wednesday morning, Goldstein said, calling it a multi-day event. More information is expected at a Wednesday afternoon briefing.