Authorities responded to the 500 block of N. Dalton Avenue at 2:43 p.m. after getting reports of a large plume of smoke coming from the garage area of the property.
"When I looked towards the house, the living room was all in flames," said Raul Favela.
When firefighters arrived, the garage and house were fully engulfed by fire. There was also apparently the sound of ammunition "burning off."
As crews battled flames, they found the bodies of three adults inside the residence.
'When they made entry into the residence, they located three deceased adults inside," said Azusa police Lt. Mike Bertelsen.
The bodies weren't removed as of 11 p.m. The coroner was not allowed inside until authorities were done removing items from the home, including assault rifles, shotguns and what police called "suspicious devices." The bomb squad was on-scene with a canine to secure the area.
"We've recovered a number of weapons from inside, approximately a dozen rifles of a variety of types, and at least 10 handguns, numerous rounds of ammunition, some black powder, and some suspicious devices," said Bertelsen.
The victims have not been identified, and it remains unknown if they died in the fire or from a crime committed beforehand. Police said the 911 calls came from neighbors; nobody from inside the home called for help.
Neighbors say the owner of the house died recently.
"The owner of the house died four months ago, so his two sons were living there with another lady," said neighbor Brianna Hernandez.
The cause of the fire is under investigation.