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

The cause of a fire at a Massachusetts assisted-living facility that left 10 people dead remains undetermined for now as officials investigate whether smoking materials and an issue with a medical oxygen machine may have been factors,