Ildiko Krajnyak (left) and Stephen Beal.
Authorities have arrested Stephen Beal, 59, a business associate and former girlfriend who was killed in a blast at a spa in California.
He was taken into custody on suspicion of possessing two "complete" improvised explosive devices and was not related to Tuesday's explosion, the FBI said.
The FBI said agents found the device while searching Mr Beal Long Beach's residence on Thursday.
Officials said they believed the explosion was "not an accident".
FBI researchers said they also found batteries, more than 100 pounds of explosives, two pistols and a rifle at Mr. Beal's home.
Mr. Beal told the FBI he was a rocket model buff. Neighbors also told local media that they witnessed Mr. Beal working on a rocket in his garage.
The FBI statement by Special Agent Evan Jesch noted that Mr. Beal "initially said he did not recognize" one of the devices. Later, he said that the device was a smoke detonator that would release the smoke to make sure the rocket remained visible in flight.
Mr. Beal has been charged with one charge of possessing an unregistered destructive device.
According to the statement, Mr Beal said his relationship with the victim, Ildiko Krajnyak, aged 48, "began to become cold because of disagreements over the exclusivity of relationships and financial problems".
He appeared briefly in court on Thursday but did not submit an appeal. Detention detention is scheduled for Monday.
FBI spokeswoman Laura Eimiller said that no further details would be available until there were federal criminal complaints.
Ms Krajnyak is a spa owner in Aliso Viejo, where the explosion happened.
Ildiko Krajnyak, 48, was killed in the blast.
State documents show Mr Beal has enrolled Ms Krajnyak as chief executive officer of a skincare company he started last year called I & S Enterprises.
Mr. Beal is the company's chief financial officer, who is based in the building that was hit by an explosion.
Her Facebook page contains some photos that appear to show she traveled with Ms Krajnyak in the past year.
Mr Beal told the FBI investigator that he and Ms Krajnyak were dated for about a year and a half.
"We do not believe this is an accident," FBI officer Paul Delacourt said on Wednesday.
"Although the damage is very large, there are some components we found at the explosion site that are inconsistent with what can be expected to be found in this business."
Beal is expected to appear in US District Court in Santa Ana on Thursday.
Damaged medical office, visible from the inside.
The Tuesday blast, which the witnesses said was like an "earthquake," also caused two people to be injured.
The blast destroyed the first floor of the medical building where the spa was located and also damaged nearby buildings.
