At least one person has died and several others are unaccounted for after an explosion destroyed three houses and damaged at least a dozen more on the outskirts of Pittsburgh, Pennsylvania, Saturday morning, authorities said.

First responders who rushed to Plum, a borough in Allegheny County, arrived to find people trapped under debris and took three people to a hospital, one of them in critical condition, Allegheny County officials said.

Another person, who has not been identified, was pronounced dead at the scene, said Steve Imbarlina, assistant chief for Allegheny County Emergency Services.

It appears the incident started when one house exploded, engulfing two neighboring homes in flames as well, according to the county. Multiple other homes were damaged with windows blown out.

Crews from 18 different fire departments converged on the scene to put out the flames and sift through the rubble as “several” people remained unaccounted for Saturday afternoon.

It’s unclear what triggered the explosion. Authorities say the cause is still under investigation.

Ring doorbell video obtained by CNN affiliate WTAE appears to show one of the homes exploding in a ball of fire, shooting up a thick plume of smoke and scattering debris in the area.