This is not parallel construction, assuming their "other reasons" for suspecting these people are legitimate and disclosed. Parallel construction is when law enforcement receives information about a crime in an illegal way or a way that cannot be disclosed, and reconstructs a plausible way in which the crime might have been discovered.

This is just building enough evidence to convict someone based on matching a photograph of a partial finger to a suspect's hand. Which sounds perfectly reasonable and legitimate (as legitimate as fingerprints can be). Just because there's not enough in the photo to match a national database doesn't mean there's not enough to be statistically certain it's a match with the suspect.