ANCHORAGE — The nail found in the truck tire of the man charged with killing two co-workers at a Kodiak Coast Guard facility likely was inserted mechanically, not picked up along a roadway, a tire expert said Monday.

The weathered nail, about 3.5 inches long and bent 7 degrees, was perpendicular to the tire tread, Gary Bolden testified. The head of the nail showed no abrasions that would have indicated it had been driven on asphalt or gravel, Bolden said.

“My conclusion was that it was inserted manually,” Bolden said.

Federal prosecutors put Bolden on the stand to poke holes in the alibi of James Wells, 62, who’s charged with murder in the shooting deaths of Richard Belisle, 51, a fellow electronics technician, and Petty Officer First Class James Hopkins, 41, on April 12, 2012.

Prosecutors say Hopkins, a nationally recognized expert in communications antennas, was unhappy that his power was ebbing at the shop where he had worked for more than two decades.

All three men were due at work at around 7 a.m. the day of the shootings. Belisle opened the shop with a security swipe card at 7 a.m. and a security camera showed Hopkins pulling into the parking lot at 7:08. Their bodies were found shortly after 7:30 a.m.

Wells told investigators he detected a soft tire on his pickup, pulled into the Kodiak Airport parking lot, examined the tire and drove home to repair it. A defense attorney said his opening statements that Wells was further delayed by chronic diarrhea after gall bladder surgery and spent 20 minutes in an airport bathroom.

Bolden works for Standards Testing Labs in Massillon, Ohio, and usually testifies on tires in civil cases. The engineer worked for Goodyear for 30 years before joining the testing lab to analyze tires and wheels and why they fail.

The FBI shipped the tire to Ohio for analysis and Bolden saw the nail below the tread in the outermost groove of the tire. Nails do not generally enter tires perpendicular to the tread, he said.

“It’s very unusual, especially for a nail of this size,” he said.

Rubber around a nail will begin to show wear. This one did not, he said.

“The material was tightly adhered to the shank of the nail,” he said.

In a static test, he pressurized the tire to its capacity of 80 pounds per square inch and let it rest for 13 days. The tire lost only 7 psi, indicating a slow leak. In a dynamic test, he inflated the tire again and ran it 24 hours on a dynamometer at 62 mph, the equivalent of 1,488 miles. It still showed only a slow leak, down to 62.8 psi.

Bolden concluded that Wells could have driven on the tire for several hundred miles before detecting it was soft. The nail head, however, showed virtually no wear, he said.

“There would be abrasion on the head of the nail if that had happened,” he said.

Also testifying Monday was John Stein, retired Coast Guard electronics technician who once supervised Wells and became friends with his family. Stein said he left a safe with his gun collection in Wells’ care while he traveled in 1996 but came back to learn that a Smith & Wesson .44-caliber silver revolver was missing with no explanation. Stein reported the loss to Alaska State Troopers.

Investigators did not find the murder weapon in the case. Robert Shem of the Alaska state crime lab testified a Smith & Wesson was one of three revolvers that could have fired bullets recovered from the scene.