According to cops, 22-year-old Michael Boatwright was arrested on a drug charge on July 5 ... and then served with an arrest warrant for first degree murder on July 10.

Detectives are calling Boatwright a gunman in XXX's murder and say they're still looking for an additional gunman.

Boatwright has a lengthy criminal past with several drug charges as well as an arrest for battery. He's also had several mental competency hearings. According to the arrest report, when he was arrested on July 5, cops discovered there was a DNA warrant out for Boatwright.

As we reported ... the Broward County Sheriff's Office also identified 22-year-old Robert Allen as a person of interest in connection with the killing of the 20-year-old rapper. Cops had said Allen was on surveillance video at the motorcycle shop in Pompano Beach where XXX was shopping before he was brutally gunned down. Allen is still wanted.