Paul Mitchell's Son Angus Cause of Death Revealed, Accidental Pool Drowning

Paul Mitchell's son's death remains shrouded in mystery -- because even though we now know how he died ... it's still unclear exactly what led up to it, and why.

TMZ has obtained the police report and medical examiner's report for Angus Mitchell's death case in Hawaii -- and the paperwork certainly sheds more light on what happened before AM passed away ... as well as what went into trying to save him.

According to the docs, cops say witnesses told them Angus had been partying with friends the night before he was found at the bottom of the pool at his Honolulu home ... but nobody had a clear explanation on what led up to him getting into the water and never reemerging.

Long story short ... the report says a driver showed up to Angus' home that morning on Jan. 3 -- when he was supposed to get picked up and taken to the airport -- but Angus was nowhere to be found. Upon a quick search, the driver discovered Angus in the water ... looking lifeless.

Cops say he dove in and fished him out, attempting to give him CPR -- and when first-responders arrived, they continued to try and do the same ... even using a defibrillator.

In the end, the life-saving measures proved unsuccessful and Angus was pronounced dead.

Cops say they did a thorough investigation and they say that after conducting interviews -- the last thing any of Angus's friends say they remembered was him heading out to the pool deck by himself, not looking under the influence or seeming out of sorts.

There's even surveillance video that was apparently taken -- and still images showing Angus in the backyard getting into the water.

Like we said ... none of Angus' friends thought anything was amiss with him -- noting he wasn't on any medication, nor was apparently fit and healthy.

The only thing witnesses cite in the report is that Angus had been drinking the night before -- but it doesn't sound like anyone was alarmed by the amount. In terms of the medical examiner -- they say Angus died from an accidental drowning ... but there doesn't appear to be any mention of toxicology. FWIW, much of the paperwork is also heavily redacted.

We broke the story ... Angus had literally been posting photos of him and his pals at the get-together the day before he turned up dead -- and he looked as happy as could be. He was the heir to his father's John Paul Mitchell Systems hair care products empire.

He was 53.