RENO, Nev. — A woman set fire to a mattress and touched off a blaze that killed six people and gutted much of a historic brick building in Reno’s downtown casino district, authorities said yesterday.

The woman was arrested on arson and murder charges in Tuesday night’s fire at the Mizpah Hotel, police said. It was the city’s deadliest blaze in more than 40 years.

About 30 people were injured, some of them after they leaped out the windows of the three-story building, which was primarily a residential hotel.

Firefighters said they did not know of anyone still missing in the fire, but they had yet to search some areas. The roof collapsed, and authorities said the building would have to be shored up.

Police Chief Michael Poehlman said Valerie Moore, 47, set fire to a mattress in her hotel room. Officials said they were trying to figure out why.

Police said 60 to 80 people were inside the 84-year-old, recently renovated hotel when the fire started. It quickly engulfed the north wing of the hotel, near Harrah’s casino. None of the downtown high-rise hotel-casinos were threatened.