TMZ Studios has a new series, "Strange & Suspicious," now airing on a dozen FOX stations, where we explore unexplained and straight-up weird stories, and now we're looking into the circumstances surrounding Riley's death.

A college student visiting Nashville with his frat brothers, Riley left his friends behind at the bar and drunkenly stumbled through the streets before vanishing ... until his body was found floating in a river, without his shoes, pants or wallet.

Riley's clothes weren't found in the water or on the river bed ... and his friends didn't chase after him when he got booted from a watering hole, and they've since lawyered up ... so they're not talking.

What's more, video shows Riley struggling to walk ... he's clearly inebriated, and at one point he falls to the ground.

The toxicology report says Riley had a .228 blood alcohol content, plus THC in his system ... but that doesn't explain how he ended up in the Cumberland River.

Authorities ruled Riley's death accidental, but it seems like there's more to the story here.

Where was Riley running to in the video??? Was he going to meet a friend, or just drunk and lost in an unfamiliar city???

Cops even talked to Riley during his bender, but in a short interaction he seemed to be fine. Was that just adrenaline, or proof something nefarious happened to him later that night?!?

We explore all the theories and conundrums at play in Riley's death on "Strange & Suspicious" ... and there's a lot to unpack.