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, however, the cause is classified as undetermined because investigators found two possible origins, officials said.

The state's deadliest blaze in more than four decades has highlighted the lack of regulations governing assisted-living facilities that often care for low-income or disabled residents.