Investigators say a fire that killed 10 people at a Massachusetts assisted-living facility was likely caused by smoking or an oxygen machine's electrical problem.

The Massachusetts State Fire Marshal revealed the possible causes of the fire that killed 10 people at an assisted living facility in Fall River.

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

The deadly fire at an assisted-living facility in Fall River, Massachusetts, was accidental, but the cause is classified as undetermined.

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.

Ten people have died after a Massachusetts assisted living facility caught fire Sunday night, making elderly residents rush to their windows to scream and beg for help as smoke filled the hallways.

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

Officials announced a preliminary investigation shows a oxygen tank malfunction or smoking may have caused the deadly Fall River assisted living facility fire that claimed the lives of 10 last week -- and the presence of medical oxygen throughout the building likely fueled the massive tragedy.