EXCLUSIVE

Lil Peep's fatal overdose is prompting the Drug Enforcement Administration to jump on the case, and we're told it has everything to do with one specific drug ... Fentanyl.

Sources close to the investigation tell TMZ ... the DEA will assist local authorities in Tucson where the rapper overdosed on Fentanyl and Xanax. More specifically, the DEA's main objective is finding out who provided the powerful opioid.

We broke the story ... cops are interested in texts allegedly sent from a woman named Mariah Bons ... who we're told went to the rapper's tour bus to see him before he died.

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