A fire that killed 10 people at a Massachusetts assisted-living facility was unintentionally caused by either someone smoking or an electrical issue with an oxygen machine, investigators said Tuesday.

An investigation into the source of the fatal fire at a Massachusetts assisted living facility last week classified the cause as "undetermined," though officials believe it was an accidental ignition

Massachusetts fire marshal reports the Gabriel House fire that killed 10 and injured over 30 was accidental, possibly sparked by a resident smoking or a mechanical issue.

The July 13 fire killed 10 people and injured at least 30. Its exact cause might never be determined, officials said.

Elderly residents – some in wheelchairs and some dependent on oxygen tanks – were blinded by deadly smoke as they tried to escape an assisted-living facility in Fall River, Massachusetts, which caught fire Sunday evening.

A resident is suing a Massachusetts assisted-living facility, claiming poor safety protocols left him trapped during the deadly fire.

Brenda Cropper, 66, died at a hospital after being in critical condition all week after the fire erupted on Sunday, officials said.

All 273 assisted living facilities must send a letter to residents and families detailing fire safety protocols after deadly Fall River fire.

All 273 assisted living facilities in Massachusetts will be required to send a letter to all residents and families detailing fire safety protocols.