Jack Mallinson, 25, a trainee solicitor and special police constable with West Yorkshire Police, was identified by NCA investigators as username winterz100, who used instant messaging app Wickr to send encrypted content.
He was arrested on 7 January last year by NCA officers who seized several of his electronic devices.
Officers found 3,710 indecent images of children (IIOC) on Mallinson’s mobile phone in categories A-C (A being the most severe) and 14 extreme pornographic bestiality images on his gaming laptop.
Mallinson was charged with five counts of making and possessing IIOC and one count of publishing obscene posts.
He pleaded guilty to all the charges at Leeds Magistrates’ Court on Wednesday (2 March) and is due to be sentenced at Leeds Crown Court on 30 March.
NCA operations manager Matthew Brooks said: “Behind every one of these images and videos is a young victim, who is powerless and exploited.
“The NCA will continue to do everything it can to bring individuals like Mallinson to justice and protect those affected by online child sexual abuse.”