The county's bomb squad and a hazmat team are preparing to search the building where the explosion occurred.
One person was pronounced dead at the scene, and three others were injured, OCFA said.
Orange County Fire Authority Chief Brian Fennessy said 70 firefighters were dispatched in response to a 1:09 p.m. Tuesday call.
The investigation is still in its early stage and authorities had begun processing the inside of the building, he said.
Authorities said they are still trying to figure out what happened and wouldn't comment on whether the incident could be related to a gas leak or explosive devices. The federal official said the information about a package-based explosion was based on a preliminary investigation.
"Anytime you see an explosion of this magnitude, it would definitely be suspicious to us and that's why we rolled out all the resources to get to the bottom of it", he said.
"There's heavy damage on the first floor corner of the building, walls and windows blown out", Capt.
The bomb was reportedly addressed to a specific person.
Sheriff's deputies were called to the same block of Mareblu at 11:35 p.m. Monday for a "patrol check", according to sheriff's records.
At a news conference late Tuesday, Paul D. Delacourt, the assistant director in charge of the F.B.I.'s Los Angeles field office, said his agency was working with the sheriff's department and the Bureau of Alcohol, Tobacco, Firearms and Explosives to investigate the blast.
Children and staff at nearby Academy on the Hills preschool initially sheltered in place while the investigation continued, but children were later evacuated to a Target store parking lot so they could be reunited with their parents.
Dong Shin, a witness, said he heard a loud boom minutes before his appointment in a nearby building. "My whole body shook".
He said it felt "almost like an quake, but a big natural disaster". Two of the injured people had unknown injuries, and the third person suffered smoke-inhalation, according to officials.