As the investigation moves forward into the cause of the deadly Ghost Ship fire last Friday — which has been deemed the deadliest structure fire in the US since the 2003 Station nightclub fire — officials are now confirming that the fire began on the first floor and sent smoke up two stairwells to the second floor, overwhelming partygoers with smoke and trapping them with flames below.

Federal agents from the San Francisco office of the Bureau of Alcohol, Tobacco, Firearms and Explosives are leading the investigation, as KRON 4 reports, and no exact source for the fire has yet been determined, though officials have pointed to a refrigerator at the rear of the warehouse that could have been to blame.

Jill Snyder, special agent with the ATF, noted that neither stairwell led to an exit, and it's been reported that the rear staircase, behind what was a stage area, was not known to most visitors and was mostly blocked off because it was used as a conduit for wires and cables. She said so far the evidence showed no signs of arson, but that most of "the occupants were consumed by smoke before they could get out of the building."

As has been reported widely, there were no smoke detectors, alarms, or sprinklers in the space, and big questions remain over who dropped the ball in inspecting the building for fire safety after years of illegal construction.

Engineers will soon be brought in, KRON 4 reports, to examine the electrical system used in the Ghost Ship space, which has been rumored by previous tenants to be highly illegal and hazardous, with daisy chains and frequent sparks.

All previous coverage of the Ghost Ship fire on SFist.