ESCONDIDO, Calif.  Darrell Littleton calls them “his guys,” but he does not trust them.

One got drunk and exposed himself to a jogger in a public park. Another was a fire captain until he molested his 13-year-old stepdaughter, went to prison and lost his wife, his job and his home. Now the man sleeps behind a drive-through restaurant.

Mr. Littleton is a parole agent, and “his guys,” about 40 in all, are paroled sex offenders. On a September morning, as he does each day, Mr. Littleton fired up his laptop computer to check on his charges; the signals from their global-positioning ankle bracelets trace dotted trails cutting through a Google satellite map. Mr. Littleton tracks them, calls them frequently and shows up unannounced to make sure they are behaving themselves. But they still struggle to stay straight.

One of his parolees recently harassed a teenage prostitute, and Mr. Littleton had to “violate him”  revoke his parole and return him to prison. Another promised Mr. Littleton that once he is off parole in a few months, and no longer subject to random drug testing, he is going to resume his marijuana habit. And before the day was over, another parolee would emerge as a suspect in a sexual assault on a 9-year-old girl. “Twenty is really the ideal caseload for my guys,” Mr. Littleton said as he drove a high-riding pickup truck on one of several parolee visits he had planned that day. “With that kind of caseload, I could spend more time in the field and less in the office. With these guys, you don’t want them to know you’re coming. You need to watch them when they don’t know they’re being watched.”

Image Darrell Littleton, a California parole agent, checks on his guys, parolees in the Escondido area. Credit... Michal Czerwonka for The New York Times

A series of high-profile crimes involving parolees in California highlight the challenges of keeping track of them in a state that discharges more than 120,000 inmates annually, more than any other.