One year after an explosion leveled a corner of the Nosler factory, operations are back to normal at the Bend bullet and ammunition plant. But the cause of the incident is still a mystery.

Triggered by a fire in an underground testing range, the blast flattened the southeast corner of the 80,000-square-foot facility on Southwest Columbia Street, scattering debris and rattling windows across several blocks. Even though the explosion came at possibly the worst time of day, right as the first shift was leaving and the second shift was coming on duty, none of the roughly 100 employees at or near the plant was injured.

Company spokesman Zach Waterman said despite an extensive investigation by the company, the Bend Fire Department, the state fire marshal and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, it’s still not clear exactly what happened.

What’s known is this: On June 2, 2010, an employee was in the ballistics lab at the underground range running tests, a daily practice.

The range consists of two buried tunnels, each about 21⁄2 feet in diameter, running the length of the building. Using a gun receiver securely mounted to a bench in the ballistics lab, employees fire through the tunnels to test the accuracy of the ammunition. At the far end of the tunnels, 100 yards from where the shots are fired, a series of microphones detects the precise location of the passing bullet.

On the day of the explosion, an employee was in the lab firing test shots when he observed a light in the middle of the tunnel, presumably at the “50-yard room,” a small room at the halfway point of the tunnels. Assuming the light was a fire, he left the lab and pulled the fire alarm at 2:02 p.m.

Waterman said investigators determined that the fire first spread toward the target end of the range, where it destroyed a ventilation fan that pulled fumes through the tunnel. With air no longer being drawn in one direction, the fire moved toward the ballistics lab. When it reached the lab, the best guess is that the additional oxygen allowed the fire to quickly expand. At 2:12 p.m., an explosion lifted the corner of the roof, ripped open the corner of the building, and pushed a 20-foot-tall concrete wall over on to a parked car.

Mysteriously, no one has been able to determine the source of the original spark, or what fueled the fire in the tunnel.

“To this day, the ignition source has not been identified,” Waterman said. “We may never know. We’ve just got to keep looking.”

Nosler has taken some new safety precautions since the explosion. An electrical outlet in the 50-yard room has been disconnected, and improved fans with dedicated power sources have been installed. The ballistics lab was rebuilt with fire-retardant materials, and methane gas detectors were installed in the tunnels.

Waterman said he and others at the company were surprised by how strongly customers reacted to news of the explosion.

“We had people call and say they were going to buy all the Nosler products they could to help us out,” he said. “We had people call to ask if they could send money — ‘No, no, no, keep your money.’ ”

Waterman said the company never considered moving instead of rebuilding. “This is our home, and it’s where we’re going to stay,” he said.