One morning his probation officer showed up unannounced.

“I took apart the computer and hid the parts under the mattress,” he said.

But Mr. DeVoss forgot to hide the mouse. He went to prison.

A 39-year-old man convicted of transporting chi ld pornography in his 20s went through a similar cycle. In his case, he was permitted to use the internet only on cleared devices, and he was explicitly prohibited from looking at pornography. He came up with hacks to bypass the software tracking him, he said.

The man, who asked not to be identified but whose probation officer verified the details of his account, said the authorities checked his hard drive and learned that he had violated his probation. He went back to prison, and when he was released he found new workarounds. Sometimes they worked, but more often they did not, he said.

How does monitoring work?

In terrorism cases, a judge usually set up the rules and then a probation officer and an F.B.I. agent watch for violations, said Mitchell D. Silber, a former director of intelligence analysis for the New York Police Department, who was a founder of a nonprofit that tries to rehabilitate extremists.

The person doing the real-time digital monitoring is typically a probation officer. (Mr. Lindh’s case will be managed by the probation office of the United States District Court for the Eastern District of Virginia.)

The approach varies from district to district, said Brian J. Kelly, a federal probation officer and cybercrimes specialist in the Eastern District of New York. Many probation officers rely on tools that track a person’s keystrokes, search terms and downloads, among other activities.

“We get a lot of false alerts,” he said. If someone under supervised release reads an article referring to “food porn” or sends an email complaining about his boss using a certain four-letter word, Mr. Kelly says he will receive a notification.