The official was briefed on the investigation but wasn't authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.
Associated Press writer Michael Balsamo in Los Angeles contributed to this report.
One person was killed and three others were injured on Tuesday in an explosion at an office building in the USA state of California, authorities said, Xinhua reported.
The explosion rocked a medical building in Aliso Viejo, California, on Tuesday.
Bomb squad investigators and FBI agents were looking into the cause of the blast Tuesday.
Fire authority spokesperson Captain Tony Bommarito later updated that three others suffered injuries and were taken to the hospital.
Authorities have not yet found explosive devices in the area, according to officials.
Mary McWilliams told The Register that she arrived for an appointment to find smoke surrounding the area and vehicle alarms going off.
". They weren't on fire, they were helping each other across the street, walking very slowly and a lot of shock, shaking, just full of ash all over their face, eyes bloodshot, just blood all over", the woman said.
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Orange County officials sought assistance in the investigation from federal agencies, including the FBI, ATF and Homeland Security. Dave Sawyer said the size of the explosion is suspicious.
He said the person who died was inside the building when the blast occurred.
He said authorities believe the explosion came from inside the building, since the doors and windows on the bottom floor were "blown out", according to The Register.
Orange County Fire Authority Chief Brian Fennesy said they received a call around 1:09 p.m.
Fire and sheriff's officials said the building appeared to be undergoing some type of renovation.
Although the cause of the blast was still unknown, Bommarito told reporters around 3:30 p.m. that there was "no immediate hazard" to the community.
There was no immediate information about the person who was killed.
Fennessy says children at a daycare across the street were not injured.
Parents whose children were evacuated can reunite with them at the parking of the Target on La Paz Road, OCFA tweeted.