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He and undercover agents shopped around a truckload of milk crates. The 11 recyclers who took the bait were arrested.

Mr. Wall expanded his operation and now works with law-enforcement officials in Orange, San Diego and Los Angeles counties.

He said his biggest raid targeted E.S. Plastics in the Los Angeles suburb of Maywood.

In April, he said, he and his colleagues recovered 24,000 pounds of ground-up plastic from crates belonging to dairies, bakeries and beverage companies.

“It took a 50-foot trailer to haul all the stolen property out of there — at least a quarter-million dollars of plastics,” Mr. Wall said.

Orlando “Alex” Bran, the owner of the company, and another person were arrested for investigation of grand theft and receiving stolen property, said Maywood police Sgt. Scott Anderson.

Los Angeles County prosecutors are considering whether to file charges. Mr. Bran, who is free on bail, declined to comment.

Individual dairies, meanwhile, are adding security staff to solve the milk crate mystery.

Alta Dena Dairy, located east of Los Angeles, hired Edmund Woods, a former police officer, to find its stolen containers.

A tip led Mr. Woods to Santee Alley in the downtown Los Angeles Garment District, where he recovered more than 300 containers from merchants using them for storage.

“We don’t take a large SWAT team in, just one or two officers for a uniformed presence,” Mr. Woods said. “We’re very polite and we go in real soft.”

Mr. Wood hopes the raids encourage college students and homeowners to voluntarily return any stolen milk crates they might be using. Alta Dena’s even has set up a “milk crate abuse” hot line at 800/457-6688 for people to surrender the containers, no questions asked.

“People look at milk crates like they’re nothing,” Mr. Woods said. “It says right there on the crate that this is private property. … It’s clearly marked so there’s no mistaking it.”