A female of undisclosed age was killed, and three people were injured in the explosion at a medical building, authorities said.
The Orange County Fire Authority said the explosion occurred from inside the two-story building at 11 Mareblu Tuesday shortly after 1 p.m. The first floor incurred significant damage.
A woman was pronounced dead at the scene, and three others were injured, OCFA said. One of those three suffered smoke inhalation.
The federal official said the information about a package-based explosion was based on a preliminary investigation. Investigators said they hadn't yet located an explosive device and were searching through debris.
The official was briefed on the investigation but wasn't authorized to discuss the matter publicly and spoke to the AP on condition of anonymity.
Agents from the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives were at the scene Tuesday night.
Sheriff's officials said the size of blast made it suspicious.
The person who died was inside the building at the time of the blast, officials said. Some reports indicated the building might have been under construction work at the time.
Authorities initially believed a car had crashed into the side of the structure but later said that was not the case.
The FBI said there was no initial indication of terrorism, but the agency would continue to investigate.
The U.S. Postal Service was also brought in to the investigation. The USPS later issued a statement: "The U.S. Postal Inspection Service, the federal law enforcement branch of the U.S. Postal Service, has determined the package in question did NOT go through the Postal Service The U.S. Postal Inspection Service will continue to work closely with our law enforcement partners in this ongoing investigation."
In an evening briefing, Orange County and federal officials said no cause had been definitively determined or ruled out, whether a gas leak or an intentional act. They expected a lengthy investigation to determine the cause.
They said they were not actively looking for any suspects.
The scene was declared a second-alarm fire and a hazmat situation. OCFA said a bomb squad was sent in to assess the situation and investigate the cause of the blast.
A nearby preschool was ordered to evacuate. Babies were seen being wheeled out of the facility in cribs, and toddlers held hands with each other and firefighters as they walked to a nearby Target for safety.
Portions of Pacific Drive were shut down in the area due to the incident, according to the city of Aliso Viejo.
ROAD CLOSURES: Due to a structure fire/explosion on Mareblu, there’s a full closure of eastbound Pacific Park to the Aliso Plaza and full closure Pacific Park westbound Moulton. Please take alternate routes. Stay tuned for more information. #alisoviejo pic.twitter.com/STWN0MVst4— City of Aliso Viejo (@alisoviejocity) May 15, 2018
The Associated Press contributed to this report.