Long Beach, CA – The bodies of three people who may have been overcome by carbon monoxide fumes were found in a Long Beach apartment Monday evening, the coroner said.

The bodies were discovered around 7 p.m. on the 1100 block of Gardenia Avenue after officers received reports about a foul odor coming from one of the units.

Although the cause of death is unknown and will be officially determined by the coroner’s office, a large portable propane heater was located inside the apartment, according to Long Beach police.

It appears the residents were using it for warmth, but investigators say those types of heaters are not intended for indoor use and can produce deadly levels of carbon monoxide if the area is not adequately ventilated.

The victims were only identified by the coroner as a 50-year-old man, a 30-year-old woman, and a man whose age was unknown. Their names were withheld pending notification of family members.

The investigation is ongoing.