My understanding is they found hundreds of photos and numerous videos when they searched his NY residence with a search warrant. Everything they found became admissible evidence at that point. Every person they can identify that is in a video or photo with a kid will still be investigated... Since I believe this whole thing was nothing more than a big blackmail operation from the very start, I have to assume the pictures and videos will clearly show adult people with the child victims.
Two words: Probable cause.
Probable cause to investigate further. Probable cause to obtain search warrants. Probable cause to detain and question a suspect.
They are not supposed to search for it, but if it turns up...