Brian Fanelli was his police agency’s computer maven, and in his spare time he taught children at a Roman Catholic church in northern Westchester County how to look out for sexually abusive behavior.

On Monday, Mr. Fanelli, the short-lived chief of the Mount Pleasant Police Department, pleaded guilty to one count of possession of child pornography at United States District Court in White Plains. The criminal complaint against him said that using two home computers from October 2013 through January of last year, he swapped photos and videos of children as young as 7 engaged in sex acts, sometimes forcibly.

The complaint said he told law enforcement officers at the time of his arrest that he began viewing child pornography “as research for the classes he was teaching” and soon began collecting photos and videos “for personal interest.”

Mr. Fanelli’s arrest by the Department of Homeland Security, which has enforcement powers because many of the materials originated in Europe and Asia, led to a total of 69 other men and one woman being arrested on related charges, including producing and distributing child pornography. They included two registered nurses, a New York City police officer, a Brooklyn rabbi, a Boy Scout leader, an airline pilot and an architect.