LOS ANGELES, May 15 (Xinhua) -- One person was killed and three others were injured on Tuesday in an explosion at an office building in the U.S. state of California, authorities said.

The explosion occurred on the first floor of a medical building at 11 Mareblu in Aliso Viejo in Orange County around 1:10 p.m. local time (2010 GMT), prompting a large-scale response by fire and sheriff's officials.

"We have not determined what caused the blast," said Carrie Braun, a Public Information Officer at Orange County Sheriff's Department.

"The investigation is ongoing," Braun told Xinhua by telephone.

All nearby buildings including a preschool were evacuated by officials in the wake of the explosion.

FBI spokesman Mike Gifford said the cause of the explosion is still unknown and the agency has sent resources to help with the investigation, which is being led by Orange County Sheriff's Department, local news outlet City News Service reported.

Unconfirmed initial witness reports said that the blast may have been a gas explosion, but Orange County Fire Authority Capt. Tony Bommarito was quoted as saying by City News Service that it is rare for such gas-fueled blast to occur in a commercial building.