Scott was arrested for misdemeanor marijuana possession in a Dallas suburb on March 14, and though he was first taken to a local hospital for reportedly acting erratically ... he was later transported to the Collin County Jail in McKinney.

It was there Scott began "exhibiting some strange behavior" according to Collin County Sheriff Jim Skinner, and officials say he was subsequently restrained, pepper-sprayed and had a spit mask put on his face by detention officers.

According to reports, sometime during the several hours of being restrained to a bed, Scott became unresponsive and was later pronounced dead at a hospital. Skinner called it a "profound tragedy" at a news conference.

But, Collin County's chief medical examiner, Dr. William Rohr, is calling it a homicide. According to Rohr, his findings show Scott's cause of death was "fatal acute stress response in an individual with previously diagnosed schizophrenia during restraint struggle with law enforcement."

Scott's family says he had schizophrenia and was experiencing a mental health crisis while in police custody, and family attorney Lee Merritt claims Scott was using marijuana at the time of his arrest "to self-medicate after being two years diagnosed with schizophrenia."

Merritt tells TMZ ... he got the news about Marvin's death being ruled a homicide around the same time he learned the suspects in Ahmaud Arbery's killing were being charged with federal hate crimes, so it was a bittersweet day.

Lee, who represents Arbery's family as well, adds ... "I now have another uphill battle, having to fight the system for justice. Again, the cycle continues."

Worth noting -- the Collin County sheriff announced earlier this month that 7 detention officers who were allegedly involved in Scott's death have been fired for violating department policies and procedures.