Assisted living centers are rare in Mass, but many families still don’t know what they offer — or what they don’t.

The Massachusetts assisted-living facility where a fatal fire killed nine people was caring for dozens of aging residents reliant on wheelchairs and oxygen tanks, but it lacked the safety measures and most of the staffing requirements that are commonplace in nursing homes.

Investigators are looking into a number of issues including the possibility that the fire was caused by smoking near an oxygen tank.

There’s no doubt in my mind that that fire was destined to kill 50-plus people,” Fall River Fire Chief Jeffrey Bacon said Wednesday.

A Massachusetts assisted-living center where nine residents died in a fire was cited for failing to immediately report more than two dozen health and safety incidents.