The blast was reported just before 2 p.m. at the center located at 1866 San Miguel Drive. Initial emergency dispatch traffic described the situation as an explosion with evacuations triggered by a gas leak, fire officials said.

Both people hurt were employees of the facility, according to firefighters. The extent of their injuries was not immediately disclosed, and their conditions were not available Monday evening.

The building's fire sprinkler system activated during the incident, but firefighters said the structure itself sustained no damage. Crews remained on scene to secure the area and confirm there was no lingering hazard.

Fire officials did not release additional details about what caused the water heater to fail or whether the facility was fully reopened. The assisted living center serves an older adult population, and such facilities typically house residents who may have limited mobility, making emergency evacuations especially sensitive.

Aerial footage showed emergency vehicles clustered around the facility shortly after the call came in.

Monday's explosion adds to a string of recent emergency responses across the East Bay, and it raises questions about the upkeep of mechanical systems in residential care settings, where residents often depend on staff to respond quickly to sudden hazards. Fire district officials have not said whether the water heater had been recently serviced or inspected.

Anyone with information about the incident is asked to contact the Contra Costa Fire Protection District.