The Baton Rouge PD tells TMZ Sports ... they still do NOT have a suspect in the murder of La'el Collins' ex-girlfriend -- but claim they don't believe the NFL star had anything to do with her death.
Collins' draft stock plummeted back in May when he was questioned in connection with the murder of Brittney Mills ... who cops said dated Collins before she was killed on April 24th. Mills was 8 months pregnant when she was shot and killed in her apartment. A paternity test reportedly ruled Collins out as the father. Collins eventually signed a deal with the Dallas Cowboys.
Collins was never named a suspect -- but did speak with cops as part of the murder investigation
Now, a rep for the BRPD tells us the case has essentially gone cold -- with no official suspects and no promising leads to pursue.
The BRPD says they don't believe Collins was connected with her death in any way -- and say he's no longer a focus of the investigation.
Cops say they are frustrated because the investigation hit a dead end because officers have been unable to unlock Mills' cell phone, which they believe might contain clues to the murder.