EXCLUSIVE

Lil Peep's death is now the subject of a police investigation and some very troubling private messages are under the microscope ... TMZ has learned.

We've learned the Tucson PD has opened an investigation into Peep's apparent OD from less than 2 weeks ago.

Law enforcement sources tell TMZ ... police are interested in texts allegedly sent from a woman named Mariah Bons ... who we're told went to the tour bus to see Peep shortly before he died.

She allegedly sent a message to a friend that reads, "GBC [Lil Peep] high af because of me and my friend lol."

Bons also allegedly texted, "He passed the f*** out my brother called and I tried to get him to say hi to my brother but he wouldn't wake up ..."