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Newly declassified documents reveal that Gen. Leslie Groves—director of the Manhattan Project, the top-secret operation that built the atomic bomb during World War II—misled Congress and the public about the effects of radiation. He did so initially out of ignorance, then denial, and finally, willful deception.

The documents also show that some scientists in the project, including J. Robert Oppenheimer, director of the Los Alamos lab where the bomb was first tested, kept mum about Groves’ lie rather than dispute him or confront the general directly.

The cache of documents—the latest in a series of once secret and top-secret material about the A-bomb obtained over the years by the National Security Archive, a private research organization at George Washington University—was released on Monday, within days of the 78^th anniversary of the bombings of Hiroshima and Nagasaki and in the wake of the release of Oppenheimer, the wildly (and deservedly) successful film that has grossed $500 million since its hit theaters just three weeks ago.

One of the new documents the archive obtained is a memo by four scientists, titled “Calculated Biological Effects of Atomic Explosion in Hiroshima and Nagasaki,” dated Sept. 1, 1945. (The bombs were dropped on Aug. 6 and 9 of that year.) Until this memo was written, it had been assumed the A-bomb’s victims would be killed by its blast and its heat. But this memo concluded that at least some of the deaths had been caused by radioactive fallout, days or weeks after the explosions.

And yet, the day before the memo’s date, at a press conference in Oak Ridge, Tennessee, Groves said radiation had caused no deaths and that claims to the contrary—some published in Asian newspapers—were “propaganda.” In a memo to Oppenheimer, George Kistiakowsky, the Los Alamos scientist who coordinated the biological report, said that Groves had “stuck his neck out by a mile,” so he hesitated to pass the study along.

Even by then, enough was known about radiation poisoning to have made Groves stop short of dismissing the claims so strongly. The archive’s documents show that, back in April, three months before the first test of the bomb in New Mexico, medical experts with the Manhattan Project warned of a toxic “cloud” that could spew “radioactive dust” over a wide radius for “hours after the detonation.” Some urged Groves to evacuate the area around the test site, which he resisted, not wanting to attract media attention. One scientist remembered years later that Groves “sniffed” at the warning and said, “What’s the matter with you, are you a Hearst propagandist?” (Hearst was the leading newspaper chain of the day, often specializing in sensational reports.)

On July 21, five days after the test, Stafford Warren, the Manhattan Project’s chief medical officer, wrote to Groves that “the dust outfall from the various portions of the cloud was potentially a very serious hazard over a band almost 30 miles wide extending almost 90 miles northeast of the site,” adding that there was still “a tremendous amount of radioactive dust floating in the air.” (Recent studies, based on computer modeling, suggest that radioactivity from the first atomic test spread much farther, affecting 46 states and parts of Mexico and Canada.)

Yet Groves ignored Warren’s findings.  On July 30, in a memo on the likely effects of an atom bomb dropped on Japan, he wrote Gen. George Marshall, the U.S. Army chief of staff: “No damaging effects are anticipated on the ground from radioactive materials.” (This was a deceptively written sentence: at the time, few thought much fallout would linger “on the ground,” but it was widely known that it could rain down from the sky and scatter across the air, which humans could breathe or soak in.)

Groves’ awareness of this danger is clear in an Aug. 25 excerpt from his diary, in which he wants to know if it’s safe to invite the press to come survey the test site (this, more than two months after the first test). One of the scientists told him that it “wouldn’t be so safe” if the journalists stood as close as 100 feet from where the bomb had gone off. Reporters did come on Sept. 11 and were given “white booties” to protect them from possible radiation.

It’s possible that, even at this point, Groves simply didn’t believe the worst about radiation. On the same day as his diary entry about inviting reporters, he had a phone conversation with a fellow officer at Oak Ridge about Japanese radio broadcasts reporting cases of radiation sickness. Groves said this was all “propaganda” and that the sickness was more likely caused by “good thermal burns.”

Still, Groves sent a team of inspectors to the two bombed cities to determine the impact of radioactivity. He wrote Gen. Marshall that casualties from radiation were “unlikely,” but the “facts” had to be established.

This makes sense. Before the bombs were dropped, most scientists assumed that blast and heat would be the dominant effects. Radiation would be a footnote; anyone who received a lethal dose of radiation would be close enough to the explosion to die from the blast or the heat.

However, as was later discovered, the A-bomb’s “secondary effects”—radiation, smoke, fire in particular—could, under certain circumstances, spread even farther than the effects of blast and heat.

As early as the first inspectors’ report—the one that Kistiakowsky at first withheld from Groves but eventually passed along to him—there was notation of “freak survivors” within the blast radius who later died of radiation sickness.

On Nov. 27, months after the memo about the biological effects of the atomic explosions in Hiroshima and Nagasaki, Stafford Warren, the project’s chief medical officer, wrote Groves with even more definitive proof. Of the roughly 4,000 patients admitted to hospitals in Hiroshima and Nagasaki, he wrote, “1300 or 33% showed effects of radiation and, of this number, approximately one-half died.”

Nonetheless, three days later, in testimony before the Senate Special Committee on Atomic Energy, Groves was asked if there was any “radioactive residue” at the two bombed Japanese cities. Groves replied, “There is none. That is a very positive ‘none.’ ”

Groves further claimed that no one in the two cities suffered radiation injury “excepting at the time that the bomb actually went off.” He added that it “really would take an accident for … the average person, within the range of the bomb, to be killed by radioactive effects.”

Finally, in a comment that sealed his reputation among his critics, Groves said that irradiated victims who died not right away, but after some time, would do so “without undue suffering. In fact,” he said, “they say it is a very pleasant way to die.”

Groves discounted, downplayed, then denied the reports about radiation sickness because, like many at the time, he thought that nuclear weapons would be the centerpiece of U.S. defense policy (as indeed they were for the next few decades) and that the American public would rebel against them if they were seen as something like poison gas—and thus beyond a moral threshold.

By this time, Oppenheimer had recently departed from Los Alamos, but he remained on government advisory boards. Like many scientists, he had underestimated the effects of radiation, but he was now well aware of the inspectors’ studies and of Groves’ false comments. Heralded as “the father of the atom bomb,” he felt blood on his hands, as he famously confessed to President Harry Truman. But he said nothing about Groves’ lies—at least not in public.

Some were not so silent. On Dec. 6, 1945, one week after Groves’ testimony, Philip Morrison, a Manhattan Project scientist who was on the team that surveyed the bombs’ damage in Japan, testified before the same committee, citing the facts about radiation, directly contradicting Groves’ blithe assurances. Morrison went on to become a professor of physics at MIT and an activist in the community of scientists—many of them veterans of the Manhattan Project—who advocated nuclear arms control and disarmament.

Maybe someday someone will make a movie about him.

Fred Kaplan is he author of The Bomb: Presidents, Generals, and the Secret History of Nuclear War.