Due to claiming to be about a thousand people that didn't exist, the FBI is likely investigating them under a major suspicion of being related to organized crime. Incidentally, it's very, very rare that using an unlawfully acquired name isn't related to a serious crime.
If I act like any murderer on the run, the FBI are going to investigate me like any murderer on the run. Similarly if you act like any organized crime money laundering syndicate, you'll be investigated like one!
You seem to be assuming that they fraudulently used the identities of real people. Is there any evidence of that? How do you know they weren't signing up as "John Doe #14812"?
If I act like any murderer on the run, the FBI are going to investigate me like any murderer on the run. Similarly if you act like any organized crime money laundering syndicate, you'll be investigated like one!