According to a tweet from LASD, "all indications" point to the work as the origin of the fire, but arson investigators from the sheriff's department are still conducting interviews and looking into the case.
Firefighters were back on scene Friday to investigate the official cause, while more than 300 people remained displaced.
The Los Angeles County Sheriff's Department is working to help the evacuees who returned to the scene hoping for answers and access to their building.
"Nobody is going to into that building and take your stuff, hopefully there is a lot left over," a deputy reassured residents.
One man said he ran upstairs with a fire extinguisher to help put out the flames. "I started seeing the ceiling bubble up, and it fell in front of me," said Jesse Aguilar.
But Aguilar's efforts had little effect. It was too late. The fire had already spread. His place on the first floor is now a total loss just from the water damage.
"The only thing we could probably save is documents that could dry up," said Aguilar.
Evacuee Linda Haddad lived on the second floor. She went to the evacuation center at Rivera Park Friday morning to pick up things for her 3-year-old son. She said her little boy is looking for his toys.
"It's OK. We lose everything but thank God," Haddad said.
But donations have already been flowing in. "The city of Pico Rivera is here for them, we are going to support them, we are going to get through this together," said Pico Rivera Mayor Gustavo Camacho.
Many evacuees said they're just thankful to be alive.
"Everything can be replaced except for our lives you know, our lives are not replaceable. We are here one time, and I'm happy my family got out in time," said Aguilar.