By ANNIE KNOXSALT LAKE CITY (AP) â¿¿ The biggest copper heist in Utah memory has stripped more than six miles of wire from a stretch of Salt Lake City highway. The Utah Department of Transportation first noticed the theft Thursday, officials said, but they don't know when exactly thieves snatched up the wire. The thieves either disguised themselves as a construction crew or worked in the middle of the night on multiple occasions to yank wire from the median of Interstate 15, said roadway lighting engineer Richard Hibbard. "We can't keep wire in the ground to save our lives," he said, estimating that the department deals with smaller-scale thefts nearly every week. Thieves take the wire to recyclers who will pay for metal by the pound. Officials are shocked, they say, to see a theft this big pulled off on a relatively urban and highly traveled stretch of road. Billboards dot the side of the six-lane highway that's lined with warehouses, sandy dirt and red rock. "To say the least, this was an extremely risky operation that they pulled off here," said UDOT spokesman John Gleason, adding, "This is by far the largest single copper wire theft we've dealt with here in Utah." The department doesn't know exactly when the theft happened because many highway lights all over the Wasatch Valley haven't been working. Routine road maintenance in the area is falling to the wayside as workers grapple to replace the missing wire and broken metal boxes. The thieves likely used sledgehammers to smash into boxes of wires running to light poles, clipped the copper and then used cars or trucks to pull 30,000 to 35,000 feet of wire out of the ground, authorities said. The scale of the heist, they said, indicates that the thieves had sophisticated electrical know-how.