Riley Strain's family experienced a bittersweet milestone on behalf of the late college student ... a moment that left them in tears, and with a diploma in hand.

Riley's loved ones attended the University of Missouri's graduation Saturday ... accepting Riley's diploma from the Mizzou College of Arts and Science. The crowd met Riley's family with cheers and a standing O ... with the late student's name being the first one read out.

The posthumous achievement was an understandably poignant moment for Riley's parents and stepparents, who fought back tears as they took the stage.

Remember, Riley's disappearance and subsequent death captured the nation's attention ... after he had been kicked out of Luke's 32 Bridge -- a bar owned by Luke Bryan -- for apparently being too intoxicated. Riley went missing for 2 weeks, despite an extensive search.

On March 22, Riley's body was found in the Cumberland River ... confirming that he had died. Police said they didn't suspect any foul play.

Riley's family is still searching for answers on how the college student met his untimely end ... speculating in an April interview that his drink may've been spiked.

Michelle Whiteid, Riley's mom, told NewsNation's Brian Entin her son texted her about his drink tasting odd the night of his disappearance. She speculated ... "Maybe there was something in it that shouldn't have been."

Cops, however, are still saying there are no signs of foul play.

Regardless, Riley's family isn't happy with their son's fraternity brothers ... who they say didn't stop their partying plans amid R.S.'s disappearance. As they put it ... the brothers of Delta Chi -- who were with Riley the night he vanished -- didn't report his MIA status to the police until the next day.

Riley was only 22.