Law enforcement sources with direct knowledge tell TMZ ... the manager at Adam's L.A. area apartment building found his body while doing a welfare check Saturday after no one had heard from the actor since Jan. 5. We're told he was found in the bathroom.

We're told officers believe Adam had been deceased for a while at that point ... and our sources say there was a white powdery substance found on the scene, which cops believe to be drugs. Based on that, and Adam's past struggles with drugs, investigators are looking at this as a possible OD. As we previously reported, foul play isn't suspected.

Although this is what law enforcement's thinking at the moment, an official cause of death will not be known until they get back autopsy and toxicology results.

We broke the story ... Adam's family caught wind of his passing this weekend. He struggled with drugs for years -- and was even once arrested for allegedly trying to steal pills from a pharmacy. Adam was on the show "Eight is Enough" from '77 to '81. He was 54.