Law enforcement sources tell TMZ ... there were multiple cans of compressed air in Aaron's bathroom and bedroom, as well as prescription pills. It's not clear if that's what caused him to drown, but obviously, it's a big clue into what may have happened.

Our law enforcement sources say the last time anyone saw Aaron alive was 2 AM Friday. We're told Aaron's live-in housekeeper heard a knock at the door at 2 AM and answered ... she says cops had come to do a welfare check ... unclear who called them. Our sources say the housekeeper opened the door and when Aaron heard what was going on he told them to leave and ordered the housekeeper not to bother him.

We're told the housekeeper didn't see Aaron all day Friday, and Saturday morning she went to his bedroom door to see if he wanted coffee. She heard his dog inside the bedroom going crazy, so she opened the door and found Aaron's body in the tub.

Law enforcement believes Aaron's body was in the tub for a long time as there was a smell of decomposition and the water had turned an unnatural color. We're told the tub's jets were still running when Aaron's body was found.

TMZ broke the story, cops were called to Aaron's L.A. area home Saturday for a report of a drowning. Dispatch audio from the 911 call revealed a chaotic scene inside Aaron's home with dispatchers describing a "female yelling" for help. We're told that was the housekeeper.

Aaron's cause of death is currently deferred by the L.A. County Coroner's Office until toxicology results are returned ... which could take months. We're told no suicide note was found and there were no signs of foul play.

Of course, there's been an outpouring of love and tributes for Aaron since his death -- Hilary Duff said, "You had a charm that was absolutely effervescent…boy did my teenage self love you deeply. Sending love to your family at this time. Rest easy."

The Backstreet Boys also sang and paid tribute to Aaron, his brother Nick got emotional during the show in London.