A Toronto man who is on probation for a previous child pornography conviction has been arrested again. And police say the offender is getting more savvy at hiding his crimes.

Blair Evans, 53, a former Department of National Defence physicist, has gone to jail twice for child porn offences. He was arrested again Monday by members of the Toronto Police's Child Exploitation Section and Intelligence Services.

Police say that during a search of Evans' home, they found that he used a portable data storage device to hide his collection of pornographic images. Det. Sgt. Paul Gillespie says the tiny device, called a "jump" or "thumb" drive, is about the size of a key chain and contained just over 1,000 images.

"These images were horrific in nature and typically involved children between the images of two and eight years old, many of them involved in violent sex acts," Gillespie told a news conference Tuesday.

Gillespie says that the frustration for investigators is that as perpetrators get more savvy in the use of the Internet and how to store data, it makes their job that much more difficult.

Police tracked Evans for several weeks from an Internet chat room to computers at an Internet cafe and at a university library.

He was on probation at the time of his arrest and under a court order banning him from using the Internet.

Evans is charged with two counts each of: possession of child pornography; accessing child pornography; and making child pornography. He is also charged with five counts of failing to comply with probation.

Evans has already served jail time in 1999 and 2002 for similar offences. In the 2002 incident, he was accused of having over 200,000 "horrendous" child pornography photos, videos, and images on his computer.

The previous conviction stemmed from an arrest in 1996, in which Evans was picked up for downloading child porn onto his computer at work at the Department of National Defence in Ottawa.

As a condition of his probation that time, Evans was to continue the treatment for pedophilia and sexual sadism that he began in 1997 and to stay away from computer software that gave him access to chat rooms.

Gillespie said he believes that child pornographers seldom change their behaviour.

"I absolutely believe it's something which cannot be cured and often defies treatment and thus is very, very frustrating," he said.

"I have yet to see a dramatic effect (of treatment) on anyone we've arrested."