Baroni's girlfriend was found naked and unresponsive under a bed sheet when officers arrived at the scene, according to a police report.
The woman was reported to have noticeable bruises and markings on her face and body.
Baroni's hands were red when officers found him at the scene as shown in photographs shared by Observando En La Bahía via TMZ Sports.
The 46-year-old told police that he had gotten into a drunken argument with his girlfriend in their hotel room, which he claimed stemmed from finding out that she'd slept with another man.
Baroni said he told his girlfriend to take a shower and she refused.
The retired fighter said he then grabbed the woman by her arms and forced her into the shower, which resulted in her hitting her head.
Baroni claimed the woman was unable to get out of the tub by herself, so he assisted her and took her to bed, removed her clothes and wrapped her in a blanket after she complained about being cold.
Baroni said he then left the hotel to purchase cigarettes and beer, which he claims was requested by the woman and believed she was asleep when he returned to the hotel.
Authorities will conduct an autopsy on the woman to determine her cause of death in relation to the murder case.
Baroni competed for the UFC, PRIDE, Strikeforce, Cage Rage, Dream, EliteXC, Bellator MMA, BKFC and ONE FC during his mixed martial arts career, compiling a 16-19 record in 35 matches.