Three 6 Mafia star Project Pat’s son was killed in January and law enforcement now believes they’ve nabbed the people responsible.

Law enforcement sources working the case in Memphis tell TMZ Hip Hop that U.S. Marshals recently took a 19-year-old woman named Talia Jones into custody in Minnesota for her alleged role in the killing of Patrick Houston, Jr. …  which they believe may have been the result of a targeted robbery that turned fatal.

Jones remains in custody and is expected to be extradited to Memphis to face charges. Houston Jr. was only 21 at the time of his death.

According to law enforcement,  there is evidence appearing to show Jones and Houston knew each other before the fatal encounter, and investigators believe an unnamed juvenile co-defendant — who is believed to be Jones’ ex-girlfriend — actually pulled the trigger that ended Houston Jr’s life.

We’re told digital evidence shows Jones and the juvenile were in communication before and after the shooting.