Most departments say they do not want their volunteers to confront criminals in the act. In El Paso County, Colo., which is home to Colorado Springs, the so-called citizen patrols check out burglar alarms, but if there is any indication that a crime is under way, they are instructed to call in an actual sworn officer, Sheriff Terry Maketa said.

In Fresno, where the pilot program began last year, officials say the program was vetted by the county’s district attorney to address legal concerns. The volunteers’ shirts are a different color than the sworn officers’, and they are restricted to handling nonviolent crimes such as petty theft, stolen vehicles and vandalism that is not gang-related.

“The reality is we’ve not had any challenges yet,” Chief Dyer said.

The Fresno program has drawn a diverse roster of crime fighters, including a recent class that included an assistant golf pro, a Pizza Hut manager and Steve Aberle, a Spanish teacher with a mop of gray hair. Mr. Aberle said he went through the 11-week training course to get a taste of “the edge” of police life that he had read about in crime novels. “The whole thing is very cool,” he said.

The class also included several young men like Mr. Sarkisian who said they had volunteered as a way into a law enforcement career.

On a recent morning, Officer Kent Pichardo was training Mr. Sarkisian, part of the 40 hours that each volunteer must spend in the field with an active-duty officer. They were answering calls in Southeast Fresno, a blue-collar neighborhood where the Bulldog street gang has pockets of members. And while he looked the part  with a blue jacket over his white shirt, there was little to distinguish him for Officer Pichardo  Mr. Sarkisian seemed nervous, chewing gum, sheepishly knocking on doors and scribbling in a worn notebook. (One page was labeled “Cop Notes.”)

By the end of his shift, Mr. Sarkisian had diligently worked his way through an interview with the parent of a missing teenager. At one point, Mr. Sarkisian asked whether the girl had any identifiable marks, and the parent mentioned a tattoo of a dog’s paw.