(GAITHERSBURG, Maryland)– An explosion in a Gaithersburg, Maryland, condo building Wednesday morning sent 12 people to the hospital, two of them critically injured.

Montgomery County Fire Chief Scott Goldstein said during a press conference that four children were among those injured.

The explosion happened around 8:40 a.m. at the Potomac Oaks Condominium near the 800 block of Quince Orchard Boulevard. 

Goldstein said the blast resulted in a structural collapse and there was a “gas-fed fire” in the building’s basement that took responding firefighters about 40 minutes to contain the two-alarm fire, at which point search and rescue operations began.

The fire remained active late Wednesday morning, Goldstein said, calling it a multi-day event.