The one-alarm blaze was reported at the Delta View Apartments at 3915 Delta Fair Blvd. at 3:15 a.m., Contra Costa fire spokesman Robert Marshall said.
Crews extinguished the fire in about 20 minutes, Marshall said. He said the fire was likely the result of a flare-up from a blaze that burned in the same building Monday night.
That fire was reported at 7:33 p.m. Monday and was put out in 15 minutes, fire officials said. It was deemed accidental.
Fire crews searched the attic after the fire and removed cellulose insulation, which is highly flammable, Marshall said.
However, he said, the building had undergone remodeling, and there was more such insulation in a concealed space that was not visible to firefighters.
He said it appears embers from Monday's fire ignited the hidden cellulose insulation, sparking this morning's fire.
No injuries were reported, he said.
Monday's fire displaced 10 residents and caused an estimated $500,000 in damage, according to fire officials.