Another firefighter and a civilian were also wounded in the shooting and authorities are not ruling out an ambush in the case, according to local police.
Fire Capt. Dave Rosa, 45, a 17-year veteran and father, was shot and killed after responding to reports of an explosion at Covenant Manor, a senior living facility in Long Beach, according to officials.
Long Beach Fire Chief Mike Duree described Rosa during an evening press conference as "a staunch family man" and a "joy to be around."
"He left quite a big void and the department is definitely in mourning," Duree said.
Long Beach Police Chief Robert Luna also appeared on Monday before reporters identifying the murder suspect as 77-year-old, fellow senior resident Thomas Kim.
Kim was booked on two counts of murder and one count of arson, Luna confirmed.
A revolver gun was found by cops and is believed to have been connected to the early morning attack, he said.
And two "suspicious devices" were also detected by the Los Angeles County Sheriff's Department bomb squad who rendered them "safe," according to Luna.
Luna noted that Long Beach investigators were trying to piece together what Kim's alleged motive may have been.
"We still have questions about the devices that were found and the suspect's intent -- which we do not have a full picture of," he said.
Residents had reported a smell of gas at 3:51 a.m., and when fire units responded they saw sprinklers were activated and a window was blown out, Duree said at a news conference Monday.
The shooting was reported a short time later, at 4:08 a.m., Duree said.
Besides Rosa, another firefighter was also shot, Duree said.
Ernesto Torres, a 10-year veteran, suffered a graze wound and has since been released, officials said. Torres is expected to pull through.
A civilian, identified as "an elderly male" was also shot and was hospitalized in stable but critical condition, Luna said at the news conference, earlier on Monday.
A person of interest was in custody and suspicious devices were found at the scene along with a weapon, Luna added.
Agents from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) have responded to the scene.
"These brave firefighters went through those doors and unfortunately they were met with gunfire," Luna said, adding that the Long Beach police and fire community has "lost a family member."
Rosa leaves behind a wife and two sons, ages 25 and 16, Duree said. He had been a member of the Long Beach department for the last 17 years and a captain for the last six years -- spending the lion share of his time mentoring recruits at some of the busiest fire stations, Duree said.
"Today is an incredibly sad day for Long Beach and the Long Beach Fire Department," Long Beach Mayor Robert Garcia wrote on Twitter. "We are all praying and sending love to the families and our [Long Beach Fire Department] team."
ABC News' M.L. Nestel and Jenna Harrison contributed to this report.