Dan Barry looked out his cockpit at the snow-swept runway. With his squadron assembled at Volk Field, he was second in line for takeoff. In the bay of his F106-A jet was an 812-pound Genie missile - armed with a live nuclear warhead. The 27-year-old lieutenant believed a nuclear war had begun.
It was 1962, the height of the Cold War. Days earlier, President John Kennedy had informed the nation that Soviet missile silos had been discovered in Cuba. Diplomatic relations were strained.
Bombers were flying round-the-clock sorties, and the Air Force was on the highest alert in its history.
This was the real thing, the pilots had been told.
In the six decades since the advent of nuclear weapons, there have been no known accidental detonations. There were many near-misses - and this was one of the strangest.
On Oct. 14, 1962, American spy planes captured images of Soviet missile silos under construction on the island nation of Cuba, less than 100 miles from the continental United States.
With the world's superpowers in a nuclear staredown, the young and untested Kennedy began back-channel negotiations with the more experienced Soviet leader Nikita Khrushchev. Meanwhile, Kennedy was under pressure from his own military commanders to strike first.
On Oct. 22, Kennedy went on TV to reveal the threat to the American people. That night, the Air Force was put on DEFCON 3 - for the first time since the Defense Condition system had been established.
According to protocol, 161 interceptors carrying nuclear weapons were dispersed from their bases to small airfields around the country. That's how Barry and two squadrons from Selfridge Air Force Base in Michigan ended up at Volk.
Their mission was to intercept and destroy Soviet bombers before they could strike U.S. cities. In addition to conventional air-to-air missiles, and for the first time ever, each jet carried a single nuclear rocket.
About midnight on Oct. 25, a guard at a Duluth air base spotted a shadowy figure climbing the fence. Assuming sabotage - which war planners considered a likely scenario - the guard shot at the intruder and set off the sabotage alarm, which was wired to alarm systems at nearby bases.
But at Volk Field, the wrong alarm rang.
The squadron pilots were sleeping in the base dispensary when they heard the klaxon. They had been told the threat was real, Barry recalled. There would be no practice.
The pilots scrambled to their fighters. Their mission was to get airborne and look for Soviet bombers coming over the North Pole.
The way Barry remembers it, he was second in line to take off when he saw a truck speeding toward them, lights flashing.
It would be more than 25 years before he learned what had triggered the warning. The intruder in Duluth turned out to be a bear.
The story, outlined in declassified Air Force documents, was first reported by Stanford University professor Scott Sagan in his 1993 book, "The Limits of Safety."
Sagan, who is co-director of Stanford's Center for International Security and Cooperation, uncovered it using the Freedom of Information Act and Mandatory Declassification Review process.
Sagan uncovered multiple nuclear mishaps - or near mishaps - but said the Volk Field event jumped out "as a worrisome and bizarre incident."
While there was no actual Soviet threat, Sagan said, a nervous pilot, believing the U.S. was under attack, could have mistaken a friendly aircraft for an invading bomber. And even though the interceptors did not carry offensive weapons, a crash with an armed nuclear warhead also could have been catastrophic.
The story never gained much local notice. Anti-nuclear and peace activists picked up on it, and it proliferated - usually un-sourced - on the Internet, where it took on the whiff of urban legend.
Nobody working today at Volk Field remembers the event, said Chief Master Sgt. Greg Cullen.
Lt. Col. Tim Donovan, spokesman for the Wisconsin National Guard, which maintains the air field, said the story has surfaced from time to time, but he never knew whether it was true.
Even the men involved didn't learn the whole story for many years.
Barry, now 73 and living near Seattle, retired from the Air Force in 1986 as a colonel. Initially, he said, the pilots assumed snow had shorted out the alarm system. Later, they heard it was a drunk GI trying to sneak back onto the Duluth base.
It wasn't until Sagan called him that he learned it was a bear.
As Sagan wrote in his book, the incident would almost be comical if it weren't part of a pattern of near-misses.
"That was serious business," Barry said. "We'd never flown with a nuke on board. … It was really serious. I can remember almost expecting to see inbound nuclear missiles."