According to the Mexican outlet, Tribuna De La Bahia, police found Baroni's girlfriend naked and unresponsive in bed with a sheet covering her. The report says cops noticed bruises and markings all over her face and body ... and she showed no sign of life.

When authorities looked at Baroni's body, they reportedly noticed his hands were red. They took photographs.

As for what Phil said transpired, the retired fighter tracked down authorities and told them he got into a drunken argument with his girlfriend in their hotel room when things turned physical.

Baroni reportedly told police he was upset after finding his GF slept with another man, so he told her to take a shower -- but she refused.

Baroni claims he then grabbed the woman by her arms and forced her into the shower ... which led to her hitting her head.

The 46-year-old -- who was reportedly under the influence of drugs and alcohol -- reportedly claimed the woman couldn't get out of the tub by herself, so he assisted. She then complained of being cold, so Baroni said he took her to bed, removed her clothes and wrapped her in a blanket.

Baroni then says he went out for cigarettes and beer, per her request.

When he got back, Baroni reportedly told authorities he believed the woman was asleep. Tragically, she was deceased.

Authorities are planning to conduct an autopsy on the alleged victim to determine her cause of death.

Baroni was arrested for murder ... and is currently behind bars.

The former MMA star fought for a bunch of top promotions during his career ... including the UFC, Pride and Bellator. Baroni last fought in September 2019 ... where he lost via submission.