According to the police report, obtained by TMZ, cops say they spoke with the "Sister Wives" star Tuesday upon word that Garrison had shot himself ... and they describe what Janelle says were troubling messages Garrison had been sending to some people the day prior.

The police explain that Janelle told them Garrison had texted a group of people that the Brown family works with -- presumably as part of their show -- and his words were ominous.

According to Janelle, via the police, his text to them read ... "I want to hate you for sharing the good times. But I can't. I miss these days."

It sounds like Janelle caught wind of this message to the group -- which doesn't appear to have included her -- and she told police she then texted Garrison himself ... at which point she says they had a brief conversation for a few minutes before he stopped responding.

Janelle then contacted her other kids to see who could go check on Garrison, and his brother Gabriel volunteered. Sadly, by the time he arrived to Flagstaff ... his brother had died.

Here's the craziest part ... cops interviewed 3 of Garrison's roommates with whom he was living at the time -- and some of them said they heard a pop on Monday night ... but didn't think it was a gunshot, and none of them even checked on Garrison to see what happened.

As it turns out, it wasn't until Gabriel arrived on the scene Tuesday that he discovered his brother was dead -- and that he was holding a handgun. He called 911, and that's when the roommates say they realized Garrison had taken his own life.

The roommates say Garrison was known in the household to struggle with alcohol. Gabriel, meanwhile, said he didn't realize something was amiss with this sibling -- noting he'd recently taken on a new job at a medical facility and seemed to be doing okay on his own.

One last thing ... Janelle told cops Garrison was estranged from his father, Kody. We broke the story ... Garrison died Tuesday from a self-inflicted gunshot wound. He was 25.

RIP