In 1950, an American B-36 bomber on a peace-time training mission crashed over British Columbia, Canada carrying a Mark IV atomic bomb, a weapon comparable in size to the nuke dropped on Nagasaki in 1945. According to testimonies from the surviving crew members, they had safely jettisoned the bomb, and detonated it in mid-air before the plane went down.

The crash became famous as the very first “broken arrow,” the U.S. military’s term for an accident involving a nuclear weapon. But questions swirled for decades about whether the bomb was really detonated over the ocean—or whether it went missing somewhere in the Canadian wilderness.

Five years after using the first atomic weapons to force the surrender of Japan in World War II, the United States military was preparing for a new era of nuclear warfare with its Cold War adversary, the Soviet Union. The Convair B-36 “Peacemaker” was the first true intercontinental bomber capable of carrying nuclear weapons to any part of the world, and the U.S. Strategic Air Command (SAC) was eager to test the new planes with a real payload.

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After months of lobbying, SAC leaders were able to convince the Atomic Energy Commission to lend them a Mark IV atomic bomb without its plutonium core. The bomb still contained large amounts of uranium and conventional explosives—but it couldn’t trigger a devastating nuclear blast.

On February 13, 1950, a B-36 known as Flight 2075 took off from Eielson Air Force Base near Fairbanks, Alaska with a crew of 17. The test flight was meant to replicate a bombing run on a major city in the Soviet Union. The B-36 was slated to fly a 5,500-mile route from Alaska to Montana, then down to San Francisco, its bombing “target,” and finally landing in Carswell Air Force Base in Texas.

But things didn’t go as planned. Not long after taking off, ice began to accumulate on the bomber’s fuselage and the excess weight put tremendous strain on the engines, three of which caught fire and had to be shut down. With only three functioning engines, the B-36 began to lose altitude at a rate of 500 feet per minute.

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Captain Harold Barry and his crew acted quickly. Their first order was to ditch the atomic bomb following military protocol to keep nuclear weapons or their components out of enemy hands. But when Barry’s copilot hit the “salvo” button to release the bomb, nothing happened. He then hit it a second time, releasing the bomb bay doors and dropping the Mark IV over the Pacific, where, according to crew reports, its conventional explosives were detonated and the bomb destroyed.

Then Barry set the failing plane’s autopilot to steer it on a course toward the open ocean while he and his crew parachuted into the darkness over Princess Royal Island on the coast of British Columbia. The abandoned B-36 cruised for another 200 miles, veering from its set course and crashing into the snowy flank of Mount Kologet, deep in the inland Canadian wilderness.

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Immediately, a combined force of the U.S. and Canadian military launched a massive search-and-rescue mission involving 40 aircraft scouring the frozen coastline. Thanks to their efforts, 12 of the 17 crew members were recovered alive, including one man found dangling upside-down from his parachute in a tree with a broken ankle. But five crewmen, including the weaponeer, Captain Theodore Schreier, were never found.

The U.S. military interviewed the crew, who each corroborated Captain Barry’s report that the Mark IV was safely detonated before the crash. Meanwhile, the search continued for the wreckage of Flight 2075, the only way to confirm if the airmen’s story was true.

The U.S. Air Force search team couldn’t find a trace of the downed plane and assumed it had crashed into the Pacific. But three years later, a Canadian rescue operation searching for a missing oil prospector spotted the wreckage atop Mount Kologet.

The Air Force tried three times to send expeditions to the remote mountain crash site, but each team had to turn back due to bad weather and grueling conditions. Finally, in 1954, a small demolition crew reached the downed B-36 and proceeded to strip the plane of any classified equipment and destroy it.

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Since the demolition crew’s report was top secret, no word emerged about the whereabouts of the missing atomic bomb. Were there clues in the wreckage that the bomb had in fact been released prior to impact? In the absence of definitive proof, rumors began to swirl about the true fate of the lost nuke. At the epicenter of these rumors was Captain Schreier, the missing weaponeer.

A number of unsubstantiated claims pointed to an alternative fate for the lost bomb. First, there was a rumor that a body was found with the wreckage on Mount Kologet. What if it was Schreier’s? The weaponeer was a former airline pilot and could have attempted to fly the plane back to Alaska when the others bailed out.

Adding fuel to the conspiracy fire? A second claim that Captain Barry had seen the bomber turn sharply soon after he had leapt into the midnight sky. The story began to circulate that the bomb never left the plane and that Schreier died trying to get it back to the safety of the Air Force base.

None of the rumors were confirmed by the military, however, and over the following half-century, other adventurers and amateur investigators made pilgrimages to the Flight 2075 crash site to see what they could find and/or pilfer.

In 2003, an investigative team led by John Clearwater, an expert on Canada’s nuclear weapons program and the history of lost nukes, journeyed to the crash site to make its own assessment. At first, it appeared that most of the plane had been destroyed by the 1954 demolition crew or stolen by generations of adventurous looters.

Then they found something interesting.

While the crash and ensuing demolition destroyed much of the equipment in the bomb bay, the bomb shackle—which is what held the weapon suspended there—remained impressively intact. Clearwater and his team concluded that if the bomb had gone down with the rest of the plane, and the shackle remained in such good condition, there would have been some clear evidence of the bomb in the wreckage. But there wasn’t.

The more conventional explanation of the fate of America’s first broken arrow was likely the truth—the only remains of the detonated Mark IV rested deep on the ocean floor.

The crash of Flight 2075 may have been the first broken arrow, but it wasn’t the last. Clearwater writes that in the first 24 years of the atomic age alone, the U.S. and Soviet Union jettisoned or accidentally released 23 other lost nukes.

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