The state Department of Public Safety says its investigation indicates that thefts of copper plates from a plant in Hayden may date as far back as 2006.

It had been believed the thefts began in April 2012, but the DPS says it has executed search warrants on recycling businesses in Tucson and Nogales to look for records from transactions from much earlier.

The DPS earlier recovered $1.5 million in copper plates stolen from the Asarco plant in Hayden, but the agency now says thefts of copper may have totaled more than $10 million.