Cops Don't Believe 15-Year-Old Girl Was Complicit in 'Catfish' Triple-Murder

Cops investigating the murder of a 15-year-old girl's grandparents and mother don't believe the teen was complicit in their killings in a stunning catfishing case ... and are urging other possible victims of the man responsible to come forward.

Riverside PD Officer Ryan Railsback tells TMZ his department has no reason to believe the publicly unidentified teen who came into contact with Austin Lee Edwards last week is anything other than a victim at this time given her age and the evidence they've gathered.

RPD won't yet say whether the teen was abducted or went voluntarily with Edwards, as they don't have enough information.

As for Edwards -- a cop from Virginia who's said to have driven cross-country to meet the girl -- Railsback doesn't believe it was his first time committing a catfishing-type crime.

We're told the cops think there could be some pattern of behavior from his past that could help explain things ... so they're investigating any digital fingerprints that he may have left on his phone, computer, or any social media sites he used.

At the moment, we're told cops don't know how Edwards first connected with the teen, or what may have caused him to kill her family and set their home on fire.

Edwards was involved in a shootout with cops Friday after a 911 call came in about a distressed teen in his car ... he was killed in the gunfire, and the teen was uninjured.

Autopsy results have not yet been released regarding the girl's mother's and grandparents' cause of death.