Bobbi Kristina -- Facial Injuries Triggered Investigation

Bobbi Kristina suffered injuries to her face before she was found submerged in a bathtub -- according to a report -- and that sparked the investigation focusing on her boyfriend Nick Gordon.

As TMZ first reported ... Roswell PD started taking a closer look at Nick and Bobbi Kristina's relationship after certain injuries raised their concerns.

WSB-TV in Atlanta says sources close to the investigation tell them the marks were on her face ... and more specifically, her mouth.

As we've told you ... multiple sources say there is a history of violence in their relationship -- and their friend Max Lomas told police Nick was acting strangely before they found Bobbi Kristina.