Firefighters in Southern California found the remains of two people inside what they described as a “human-dug cave or excavation” in Northridge on Sunday, authorities confirmed.

The discovery came as the Los Angeles Fire Department (LAFD) was investigating hazardous materials around 10:38 a.m. at 8700 North Lindley Avenue.

Upon arrival, firefighters found two deceased people inside the man-made cave. A white, powdered substance was also found, LAFD said. LAFD didn’t specify what the substance was, or if it could be connected to the deaths.

The listed address is for Pacifica Senior Living, though it wasn’t immediately clear whether the deceased people were residents of the community, or where within the area the “cave” was located.

No additional details were immediately made available, and the investigation remains ongoing.