Bobbi Kristina -- Now It's a Homicide Investigation

The investigation into the circumstances that led to Bobbi Kristina's death Sunday has just been amped up ... TMZ has learned it's now officially a homicide investigation.

Law enforcement sources tell us, both the D.A. and the cops are treating the mystery as foul play, and Bobbi Kristina's boyfriend, Nick Gordon, continues to be a person of interest.

Although months have separated the incident in which Bobbi Kristina was found submerged in her bathtub from her death Sunday, law enforcement is confident there will be no problem finding a direct link between the 2, and they have felt from the beginning there was foul play.