The police investigation into the murders of rapper G$ Lil Ronnie and his 5-year-old daughter in the Fort Worth, Texas area has hit a major break ... cops say they know who is responsible, and they're calling the suspects straight-up punks!!!

The Forest Hill Police Department held a press conference Wednesday and identified Adonis Robinson and Jakobie Russell as the suspects -- Police Chief David Hernandez called them both cowards and "public enemies" ... and pointed out Robinson's sex-offender past.

The two Texas natives are said to be on the run, scared and afraid to face the music ... but Hernandez vowed police will catch them.

The two suspects are accused of staking out Ronnie and little R'Mani at a Slappy's Car Wash in Forest Hill Monday, before firing off dozens of rounds ... killing the girl and then chasing down Ronnie to finish him off.