SAN DIEGO -- Four recycling center employees were jailed Thursday on suspicion of dealing in stolen metal at their North County workplaces.

The arrests were the result of a three-month undercover investigation into increasing theft of copper wiring and other metal goods throughout San Diego County, a trend that also has been documented nationwide and internationally, according to sheriff's officials.

Taken into custody were Frederick Fohl, 48, Eduardo Guillen, 20, Daniel Kwak, 26, and James Robinson, 52. Each was booked into the Vista jail on suspicion of multiple felony counts of receiving stolen property.

Guillen is employed by Lee's Iron and Metal on Lee Road in Vista. The other three suspects work for Ben Recycling on San Luis Rey Road in Oceanside, according to police.

The charges stem from the men's purported failure to exercise "due diligence" in determining that materials they bought had been legally obtained by the sellers, sheriff's Detective Jason Vickery said.

In some cases, several of the suspects allegedly made statements to undercover investigators indicating that they knew the items were stolen, according to Vickery.

Theft of metal -- most notably, copper wiring -- has increased significantly in recent years as such materials have gained in value.

The North County sting operation was a joint effort involving the Sheriff's Department, San Diego County District Attorney's Office, state Department of Justice, FBI and Escondido and Oceanside police departments.