A criminal complaint filed by federal prosecutors state that Stephen Beal, 59, was charged with one count of possession of unregistered destructive device.
MORE: Ex-boyfriend arrested after destructive device found at Long Beach home
The 11-page complaint states that a search warrant was served at Beal's Long Beach home, where authorities found two completed improvised explosive devices, three firearms, as well as "precursor chemicals, energetic materials, e-matches, variously sized cardboard and modified rocket tubes -- all known to be chemical precursors and component parts of IEDs."
Beal made his first court appearance in this case Thursday afternoon in United States District Court in Santa Ana.
He was arrested Wednesday evening on suspicion of being in possession of an unregistered destructive device, according to a statement released on behalf of the Orange County Sheriff's Department, FBI, Bureau of Alcohol, Tobacco, Firearms and Explosives and the U.S. Attorney's Office.
"Mr. Beal is not being charged in connection with the explosion," which occurred Tuesday, the news release said. Three other people were injured in the blast -- two who are expected to survive after surgery and a third person who was treated at the scene.
Neighbors described Beal as an ex-boyfriend of Ildiko Krajnyak, the 48-year-old Trabuco Canyon resident and day spa owner who was killed in the Tuesday blast. They also said Beal was a hobbyist interested in model rockets.
"He used to build rockets and stuff. I've seen him out on the street. So he was familiar with explosives," one neighbor said.
Another neighbor said she is uneasy about the string of events that Beal is tied to.
"You could see there's a street in between our houses, so I'm kind of glad of that now," Linda Brown said.
Beal's son, Nathan Beal, told The Associated Press that his father had been in a relationship with Krajnyak-Vestil. He said they had recently broken up. State documents show Stephen Beal and Krajnyak were business partners in a skincare company based in the same building where the explosion occurred.
At the scene of the explosion, federal agents said they found a cellphone, a nine-volt battery, loose wires and duct tape - all components that can be used to make an explosive device.
One of the two injured victims, both patrons at the spa, said she saw Krajnyak pick up a box from a pile of mail. When she opened the box, it exploded, according to the victim's description in the criminal complaint.
Authorities have declined to publicly say if they believed the victim was the target of the blast.
But one official briefed on the investigation told The Associated Press the woman had been the intended recipient of an explosive package. The official spoke on condition of anonymity because the official was not authorized to disclose details about the investigation.
This is not the first time one of Beal's romantic interests died at a relatively young age. Death certificates show his wife died in 2008 at the age of 49.
The Associated Press contributed to this report.