Guilherme "Bomba" Vasconcelos -- an MMA fighter who competed in the UFC and Bellator, and dated Demi Lovato -- died by suicide, according to newly-released documents.

TMZ Sports obtained the case report from the October 15 incident ... which state cops were dispatched to an Illinois home around 9 AM after a woman reported finding Vasconcelos in the garage.

After reviewing security camera footage ... cops determined Vasconcelos took his own life the night prior around 11:45 PM -- and was not discovered until the following morning.

A search of the house recovered several vials of anabolic steroids, with two mostly empty and two unopened. A suicide note was not discovered.

According to a toxicology report ... it yielded only one positive finding -- caffeine.

Over the course of his MMA career ... Vasconcelos racked up a record of 10-6. He and Lovato were first spotted together in 2017 during New Year's Eve festivities after several months of dating.

Guilherme's uncle, Walter, mourned the loss of his nephew at the time ... saying, "We were looking forward to your visit for Christmas, which will no longer happen. Life is fleeting, and we woke up to this devastating news. What a time we live in."

RIP