A former contractor for the Washington Metropolitan Area Transit Authority pleaded guilty Friday to stealing about 30,000 pounds of copper designated for Metro tracks and selling it to a scrap yard in Charles County, authorities said.

WMATA officials launched an investigation into 64-year-old James Mulcahey after the agency received an anonymous tip that copper was disappearing from a rail yard in Greenbelt, Md., according to the Prince George’s County state’s attorney’s office.

After putting Mulcahey under surveillance, WMATA investigators watched him sell copper several times, authorities said. He was arrested after a transaction in November 2013. Overall, officials believe Mulcahey made $88,000 from selling stolen copper between March 2011 and November 2013.

Mulcahey’s sentencing is set for July 31.

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