Bexar County Sheriff's Deputy Mark McGee, a detention officer at the Bexar County Jail, was arrested by San Antonio Police officers on Friday.
McGee is accused of stealing copper from a campground at Brooks City Base.
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A San Antonio Police officer said he caught McGee, 47, in the act, and was wearing his Bexar County Sheriff's Office badge and hat.
According to a police report, McGee told the officer he thought it would be OK to take the copper because the building was abandoned.
McGee is on administrative leave and has been employed at the Bexar County Sheriff's Office since December 2010.