Demolition of the north side of the Mizpah Hotel, severely damaged from a Halloween arson that killed 12, began Thursday morning when investigators released the building to its owner.

It was unknown how long the demolition would take, as extreme care is needed so the weakened structure won't cause a catastrophic collapse, Reno police and fire spokesman Steve Frady said.

The hotel, at 214 N. Lake St., was built in three phases beginning in 1922. The south side, built in 1925, wasn't as damaged and is expected to remain, said Frady, who did not know what else would be salvageable.

Authorities believe they recovered the 12th and final victim late Tuesday from the hotel after Reno's deadliest fire. Investigators have finished documenting the crime scene, but plans for the structure were unknown. Authorities believe an ignited mattress leaned against a resident's door sparked the deadly fire.

Mizpah owner Fabio Reginato could not be reached Thursday. At the time of the fire, the building had been for sale and was being renovated. Mattresses had been in the hallway after employees had changed them out from the rooms.

Frady said the building's demolition is the owner's responsibility. Currently, city officials have classified the hotel as a "dangerous building."

The fire also indefinitely closed several businesses on the first floor, some of them new shops. Frady said all businesses sustained smoke and water damage. The shops' utilities were hooked up in a central area in the north portion of the residential hotel, where most of the fire damage occurred, Frady said.

It was unknown when those businesses would reopen.