Investigators try to determine if explosion that left woman dead was intentional

One woman was killed and three others were injured in the blast, which occurred about 1:10 p.m. in the first floor of the two-story building at 11 Mareblu, according to the Orange County Fire Authority.

It was unclear what exactly caused the explosion, but investigators think it may have been caused by a package, the Los Angeles Times reported, citing two law enforcement sources.

"I think this whole thing is suspicious just by nature," Orange County Sheriff's Department Cmdr. Dave Sawyer told KTLA on Tuesday.

However, authorities have not yet been determined if the incident was a targeted attack, as the possible device that caused it -- if there was one -- has not yet been recovered. As sheriff's officials noted, the source of the blast remains unknown.

There was much confusion about the cause of the explosion in its immediate aftermath, with fire officials at first saying it happened after a car crashed into the structure. A short time after that, they indicated there was no crash and deemed the incident a natural gas explosion.

But that information was also revised during the course of the investigation.

Federal officials from the FBI, Bureau of Alcohol, Tobacco, Firearms and Explosives and the Department of Homeland Security have been called out to assist local authorities.

"They are not assuming the investigation at this time," Sawyer explained. "They're merely assisting us based on the nature of the incident."

There was no early indication of terrorism, FBI spokesperson Mike Giffords told CNN.

No arrests have been made, nor have officials identified a suspect or suspects.

According to business owners in the building, the incident took place in the office of an aesthetician practice called Magyar Kozemetika. It is owned by a woman named Ildiko Krajnyak, and her family hasn't heard from her since the explosion, her son told KTLA in a telephone interview Wednesday.

FBI agents could be seen at the woman's home, located in a residential development in an unincorporated area next to Rancho Santa Margarita, video from the scene showed.

Neighbors said there was a lot of action overnight at the property, with 50 or more law enforcement officers descending on the quiet neighborhood and began to investigate. According to residents, the area has been under surveillance since a short time after the explosion.

Still, it is not yet known if she was the victim killed, as the coroner has not yet formally released the deceased woman's name. Authorities believe the victim was in close proximity to the explosion, while the three others who were injured were likely near her.

Their conditions were not immediately known.

The damage to the office building where the deadly incident took place was "severe," according to a fire captain. Some windows and walls were blown out, and debris landed in a nearby parking lot as well as the street.

The building, located on a cul-de-sac, has several medical practices inside.

The roof one of one of those businesses, Kennedy Chiropractic, partially caved in, images showed. No one was inside that office at the time.

A portion of a ceiling at a first-floor dental practice also caved in, according to one employee who was there at the time.

"There was a really loud boom," Hailey Everett told KTLA. "I thought that maybe there was an earthquake."

Everett was treated at the scene for a bump on her head that she received after being struck by a dislodged chunk of ceiling, but she was otherwise OK.

Other nearby businesses were evacuated, including Academy of the Hills, a private preschool located across the street.

While the office building has since been deemed safe, all businesses on Mareblu were closed Wednesday amid the ongoing investigation, according to a tweet from the city of Aliso Viejo. The street has also been closed off to through traffic.

It was not immediately known how long the closures would last.

"This is going to be a slow process," said Fire Capt. Tony Bommarito. "We have an explosion, which is not a common thing."