The fire broke out around 12:15 a.m. at the 11-unit building on the 1000 block of North Bonnie Brae Street.
Two people were killed and four others were injured, including a man who jumped from a second story window to escape the flames and a firefighter who was hurt during the rescue efforts.
Investigators say a K-9 detected an accelerant inside a unit in the building, leading them to believe the fire may have been criminal in nature.
"There was a significant amount of fire. It wasn't something where it was a slow, smoldering fire," described Capt. Jaime Moore with the Los Angeles Fire Department. "It was a considerable amount of fire on two different floors."
Officials say it took 104 firefighters nearly an hour to knock down the stubborn and challenging blaze. Homicide and arson officials are investigating the incident.
Capt. Jaime Moore of the Los Angeles City Fire Department only identified the two people who died as a 90-year-old woman and a man in his 20s.
"My understanding is that one was on the first floor and one was on the second floor," Moore said.
Jose Amaya says the 90-year-old victim was his mother, Rosa Fonseca. Amaya got a phone call in the morning with the bad news.
"They told me my mother died. I didn't even know," said Jose Amaya.
Twenty-five people were displaced because of the blaze. Rayna Valomares, 14, was sleeping in a first-floor apartment when the chaos broke out. Her mother was one of the four people who were injured.
"I was really, really scared because I didn't know if I was going o be able to come out," Valomares said. "I didn't want to come out because I was really, really panicked."
The second story and the attic of the building were completely destroyed. Moore says the 4000-square-foot house was divided up into 11 separate rental units, some totaling just 200-square-feet and containing no bathrooms.
Crews are also looking into whether the apartment building was up to code, including fire exits, smoke alarms and security bars.