Bobby Brown Jr.'s drug-related death raised a lot of red flags with the police ... which is exactly why we're told they opened a criminal investigation that's now in the hands of prosecutors.

Law enforcement sources tell us ... the LAPD began looking at this as a possible crime pretty much from the outset due to the fact narcotics were involved, not to mention the specific type ... Percocet and coke. Officers quickly caught on to the drugs after talking to people Bobby was around shortly before his body was found.

We're told cops know street drugs like that are often cut with fentanyl -- which is cheap and easily hooks users. Sure enough ... that turned out to be the case for Bobby Jr. A toxicology test did, indeed, find the lethal substance in his blood.

The case is now in the hands of the L.A. County D.A.'s Office to determine if criminal charges should be filed or if the case should be kicked back to cops for additional investigation. We're told no arrests have been made ... yet.

Our sources say investigators interviewed Bobby Jr.'s girlfriend and other witnesses who saw him before his death. We're also told there were no drugs or paraphernalia found at the scene when his body was recovered.

We broke the story ... Bobby was found dead in his home in November. Like we mentioned, an autopsy showed he died from a toxic combination of alcohol, cocaine and fentanyl. His death was listed as accidental.

This is the second child Bobby Brown has lost -- and the second to have drugs found in their system. Bobbi Kristina was pulled off of life support following a bathtub accident in 2015. She, too, had cocaine and alcohol in her system.

Of course, his late wife, Whitney Houston, also died a drug-related death ... she had drowned in a bathtub, and cocaine was listed as a contributing factor.