LOS ANGELES, May 16 (Xinhua) -- The deadly explosion that killed one woman and injured three others at an office building in the U.S. state of California was not an accident, said authorities on Wednesday.

"At this time we ruled out the potential of this being an accident due to the condition of the victim," said Orange County Undersheriff Don Barnes at a news conference.

The female victim of the explosion has been identified as 40-year-old Ildiko Krajnyak. She is the owner of a day spa inside the two-story office building.

"Three search warrants were served early this morning," Barnes added, noting that authorities are investigating the incident as a crime but no one has been arrested in regard to the investigation.

"At this point, our working theory is that this explosion was caused by a device," said Paul Delacourt, assistant director of the FBI's Los Angeles office.

Evidence has been sent back to the FBI's lab in Quantico, Virginia, for testing. However, it remains unclear how the device had been delivered to the scene.

The explosion, which occurred on the first floor of the building at 11 Mareblu in the City of Aliso Viejo in Orange County around 1:10 p.m. local time (2010 GMT) on Tuesday, prompted a large-scale response by local and federal officials.

All nearby buildings, including a preschool with more than 100 children, were evacuated in the wake of the explosion.

All businesses on Mareblu are closed Wednesday and the street is closed to traffic while the law enforcement investigation continues, according to the City of Aliso Viejo.