6:15 p.m.Fire investigators have closed their investigation of a fire that killed four in Napa last week, but did not identify a cause for the blaze that killed a Napa grandmother, her two grandsons and her boyfriend.

“The cause of the fire is undetermined, meaning we don’t know what started the fire,” Napa Fire Marshal Darren Drake said. “Our fire investigation is closed.”

The fire broke out in the early morning hours of Feb. 4, in the living room of the single-story wood-frame house in the 1400 block of Vale Avenue in Alta Heights.

“The fire started in the living room by the front windows. Within that area is the point of ignition. Something first ignited the fire, but we don’t know what,” he said.

When firefighters arrived they found Aurora Castañon, 58, Eutimio Torres, 37, Mathew Castañon, 12, and Giovany Mendez, 4, unconscious in the front bedroom, adjacent to the living room. All but Mathew were pronounced dead at the scene. He was taken to Queen of the Valley Medical Center where he died.

Drake said the wall heater in the living room was not a contributing factor. “It was turned off. There were no other heaters in the area of origin.”

He said two smoke detectors were found. One in the living room and another just outside the bedroom, where the victims died. “However, we have not been unable to determine if they were working at the time of the fire.”

A professional engineer was also called in to check out the appliances and electrical components in the house.

“He came up with nothing. Everything was in working order,” Drake said.

The Christmas tree was still up in the living room, but is not the cause of the fire.

“It certainly contributed to rapid spread of the fire and its intensity,” he said.

Fire investigators believe the lights on the tree had been removed. “We found some Christmas tree lights in a corner of the house,” Drake said.

A funeral for four victims was held Saturday. Castanon and her grandsons were buried at Tulocay Cemetery. Torres’ body was sent to his native Mexico for burial.