A Seattle man convicted in what is believed to be the biggest metal theft in state history was sentenced Friday to about 12½ years in prison.

A King County jury last month convicted Donald Turpin of stealing 4.3 miles of copper wiring from the Sound Transit light-rail system between November 2010 and August 2011. He was found guilty of second-degree burglary; first-degree theft, with a metal-theft aggravator; first-degree trafficking in stolen property; and leading an organized-crime operation.

A co-defendant, Lee Russell Skelly, 45, pleaded guilty to first-degree theft and was sentenced this month to 45 days of work release. Had Turpin also pleaded guilty, his sentence would have been significantly diminished, Turpin’s defense attorney, Craig McDonald, said.

During the sentencing Turpin, 55, asked the court for leniency because of his age. If the sentence was long, he said, “it is very unlikely I would ever be released; I would die in there.”

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According to charging documents, Turpin and Skelly stole the wire from the elevated light-rail tracks between the Rainier Beach and Seattle-Tacoma International Airport stations by getting into the interstitial, an area on the underside of the elevated tracks.

After breaking in using wrenches and ratchet straps, the thieves walked for miles underneath the tracks with access to several types of wires.

The documents say the men used bolt cutters to cut the wire and then dropped segments to the ground, where they would pick them up later in the night.

The men apparently worked up a thirst while stealing the wire and brought Gatorade bottles with them into the interstitial. Those bottles, which they left behind, helped investigators after Sound Transit discovered the theft in May 2012.

DNA evidence from the Gatorade bottles and a latex glove worn by Turpin were key in the investigation, along with numerous interviews with the suspects and their associates, according to charging documents.

Turpin had a state-issued business license that allowed him to scrap the metal with little, if any, scrutiny by scrap-metal buyers. Upon closer investigation of recycling-yard records, police found Turpin had visited recycling yards day after day selling copper. He made approximately $50,000 in profit in 67 different transactions scrapping metal, according to charging documents.

Replacing the 55,000 pounds of copper wire cost more than $1.3 million, the prosecutor’s office said. The copper itself was worth more than $200,000.

Presiding Judge Dean S. Lum said that the scope of the theft and Turpin’s criminal record were central in his sentencing decision. He also voiced concerns over safety, as the copper wire was in place as a safety precaution to protect against excess voltage.

“Not only was this wrong, but it endangered the public,” Lum said “The wire was there for a reason.”

Erin Heffernan: eheffernan@seattletimes.com

or 206-464-3249