BOSTON - A former civilian employee of Hanscom Air Force Base was arrested Thursday after investigators said they found more than 3,000 encrypted images and videos of child pornography, including a manual titled "How to Practice Child Love," on his computer, in a case that had ties as far away as Australia.

William Gates, 41, of Burlington, was charged with one count of possession of child pornography. He is being held until a detention hearing can be scheduled for Friday, said Christina DiLorio-Sterling, spokeswoman for U.S. Attorney Andrew Lelling.

Federal investigators were initially tipped off by Australian authorities about an investigation of an online child pornography site. Overseas agents alleged Gates posted photos and comments of a boy on a field trip, who was about 6, she said.

"Australian and U.S. authorities were able to trace the account activity to IP (internet protocol) addresses assigned to Gate's home in Burlington and to Hanscom Air Force Base where Gates was a civilian employee at the time," she said.

Federal investigators first interviewed Gates in December and he allegedly admitted he took the photographs while chaperoning a trip to the zoo with his son's elementary school. Soon after he gave agents permission to search some of his computers and other devices.

In the initial search, agents found 51 images of child pornography. They then received a search warrant in January and found more than 900 child pornography files. He was then indicted by a Middlesex grand jury for possession of child pornography.

In August, investigators conducting a forensic analysis of one of Gates' laptops turned up a hidden and encrypted volume of about 2,900 more images and 70 videos in the computer. The examination also turned up a document which serves as a guide to teach adults how to practice sex with children called, "How to Practice Child Love," she said.

If convicted Gates faces a maximum of 20 years in prison, a lifetime of supervised release and a fine of up to $250,000

United States Attorney Andrew E. Lelling and Peter C. Fitzhugh, Special Agent in Charge of Homeland Security Investigations in Boston, announced the arrest on Thursday. The Burlington Police Department assisted in the investigation. Assistant U.S. Attorney Anne Paruti, Lelling's Project Safe Childhood Coordinator and a member of the Major Crimes Unit, is prosecuting the case.