A USA official briefed on the investigation, speaking on condition of not being identified by name, told The Associated Press the explosion is being investigated as intentional and investigators believe the blast may have come from a package.
The explosion Tuesday badly damaged a two-story building in the city of Aliso Viejo and injured three other people, including two who remain hospitalized.
But that information was also revised during the course of the investigation. Cause of explosion not determined so far, but it was prompted by a device; unclear how device was delivered.
The Tuesday afternoon blast occurred in Aliso Viejo, which is about 50 miles south of Los Angeles.
Nothing indicates there were any threats made before the blast, which appeared to be concentrated in a suite on the first floor of the office building, Sawyer said.
Associated Press writer Michael Balsamo in Los Angeles contributed to this report.
Investigators from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives assisted Orange County Sheriff's detectives Wednesday morning at the scene of an explosion in a medical building in Aliso Viejo, where a woman was killed and three other people were injured.
Mary McWilliams told the Orange County Register that she arrived for an appointment to find smoke surrounding the area and vehicle alarms going off. Fire officials say a third victim had smoke inhalation.
The Orange County Sheriff's Department said Wednesday investigators have not come to any firm conclusions.
Two survivors had critical injuries "that were consistent with an explosion, but not necessarily consistent with a bomb", said Carrie Braun, a spokeswoman for the Orange County Sheriff's Department.
Trump unloads on DHS chief over immigration
During the meeting, Trump yelled about the United States' porous border and said more needed to be done to fix it. Trump nominated Nielsen to lead the U.S.
Orange County sheriff's officials said late Tuesday night that they had not found a specific explosive device and were continuing to sift through the debris left from the explosion. Authorities believe the victim was in close proximity to the explosion, while the three others who were injured were likely near her.
"And (I) saw two women come out, full of blood, hair singed, just glass stuck to their hair, glass stuck to their bodies", one witness told CBS2.
Their conditions were not immediately known.
Federal, state and local authorities, including the FBI and the Department of Homeland Security, are investigating the blast, which occurred on the first floor of a building that was under renovation.
"We can not say exclusively this explosion was intentional", Sawyer said.
Two officials have told The Associated Press that an office in a Southern California medical building was targeted by an explosive package.
Orange County Fire Authority Chief Brian Fennessy says 70 firefighters were dispatched in response to a 1:09 p.m. Tuesday call from Aliso Viejo. "I thought it was a garbage truck but it was a building that smashed", said Kingston, Dik's 6-year-old son.
Although the cause of the blast was still unknown, Bommarito told reporters Tuesday afternoon that there was "no immediate hazard" to the community.
Children at a day care facility across the street from the building were evacuated.