The FBI said Wednesday that investigators found components for an explosive device at the location in Aliso Viejo of a blast that killed a woman and wounded three others.
An explosive device went off inside the day spa shortly after 1 p.m. Tuesday.
"We do not believe at this time that this was an accident," Undersheriff Don Barnes said at a news conference on Wednesday.
Paul Delacourt, assistant director in charge of the FBI's Los Angeles field office, said the "working theory is this explosion was caused by a device," and he reiterated that investigators "do not believe this was an accident."
"There are some components we have located at the scene that are inconsistent with what one might expect to find at this business," Delacourt said.
He said those items were being shipped to the FBI's crime lab for analysis.
Despite various media reports suggesting the explosive device was somehow delivered to the building, Delacourt said, "At this time, we do not know how the device arrived at the location."
He urged anyone who might have information about the blast to call a hotline at (800) CALL-FBI, or (800) 225-5324.
The news comes as officials identified a Trabuco Canyon woman was as the woman killed in the explosion which investigators confirmed they "do not believe this was an accident." No arrests were immediately reported.
Ildiko Krajnyak, 48, a beautician who owned a day spa called Magyar Kozmetika inside the building at 11 Mareblu.
The business is on the first floor of the building. FBI agents conducted a search at Krajnyak's home on Drover Court in Trabuco Canyon overnight and were seen carrying several boxes out of the house.
Investigators also served a search warrant at a two-story home in Long Beach, Barnes said. Barnes did not provide any details of the warrants, but NBC4 reported from the scene in Long Beach that a man was questioned at the scene. He has not been named as a suspect in the bombing.
Neighbors say the man who lives at the home is a widower with three adult children. He's an investor who works part time as an actor, they say. He recently broke up with his girlfriend.
Neighbors say they've seen the homeowner building large rockets in the driveway, which he launches out in the desert. One frequent visitor to the neighborhood says her kids used to play with the man's grandchildren.
"They have a pool and they swam with his grandkids, but we didn't want the kids there anymore," said Sicily Naso, a neighborhood visitor.
The investigation put neighbors on edge.
"I just wanted to make sure my house didn't blow up and everyone was safe," said Sophie Cauvin-Tellier.
Authorities initially responded to the building at 1:10 p.m. Tuesday on a report that a car had crashed into the building, but Orange County Fire Authority Capt. Tony Bommarito and sheriff's officials said later that was not the case.
Bommarito said the blast occurred inside the building, and the woman who died was in close proximity to the explosion. Authorities did not actually enter the building until late Tuesday, trying to ensure there was no danger of additional explosions.
Outside the building, a massive contingent of sheriff's deputies and fire crews amassed, joined by federal agencies summoned to assist with the probe.
Sheriff's deputies snapped photos of vehicles that were leaving the scene throughout the afternoon. Bommarito said one of the injured people suffered from smoke inhalation. The other two victims, both women, were believed to be customers inside the day spa when the explosion occurred, Barnes said.
Authorities said they remained hospitalized and were in surgery Wednesday but were expected to survive. 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. No children were injured, officials said.
Video from the scene showed teachers leading the children along a sidewalk toward the Target. Other people could be seen wheeling cribs down the street as the area was evacuated.
A woman who was on her way to an appointment in the area said she arrived just after the blast occurred, and she saw fire on the roof of the building.
"Then everyone started running out of buildings everywhere with looks on their faces of just horror," the woman said. "... I saw two women come out full of blood, hair singed, just glass stuck to the hair, glass stuck to their bodies. Their skin was burned and peeled back, and they were just in shock. "...
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.
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. Barnes said the response was a "deputy-initiated" check of the property at 15 Mareblu.
A source familiar with the case said, however, that a bomb threat was being investigated. It is unclear how the threat was conveyed, the source said.
Eric Leonard, Hetty Chang, Vikki Vargas, Patrick Healy, Angie Crouch, Jonathan Lloyd and Jason Kandel contributed to this report.