FARGO — During the final seconds of the countdown, most of the observers in the New Mexico desert laid down with their feet toward a firing tower that rose 100 feet above the Alamogordo Bombing and Gunnery Range.

At 5:30 a.m. on July 16, 1945, the Manhattan Project’s Trinity atomic bomb detonated, unleashing a 19-kiloton explosion, melting asphalt and sand into green glass and sending radioactive material billowing 50,000 to 70,000 feet into the atmosphere.

The blast marked the beginning of the nuclear age — and the beginning of a decades-long era of atmospheric nuclear testing that deposited radioactive fallout throughout the United States and worldwide.

The thundering Trinity test — the climactic scene in the recently released movie “Oppenheimer” — showered fallout on 46 states, including North Dakota, Minnesota and South Dakota, within 10 days.

A study using state-of-the-art computer modeling drew upon newly available data sets to reach its findings and also computed cumulative deposits of radioactive fallout in the United States for the first five days after each of 93 atmospheric nuclear weapons tests at the Nevada Test.

The study, led by Princeton University researcher Sebastien Phillippe, is the latest to estimate cumulative fallout deposits from U.S. atmospheric nuclear tests.

“Our results demonstrate the significant impact of Trinity, the first nuclear weapon test, on the overall deposition density in New Mexico and across the contiguous U.S.,” the study authors wrote.

Despite their distance from the Nevada Test Site, fallout levels were notably high in North Dakota, Minnesota and South Dakota, leaving lingering questions about cancer and other health effects.

The Forum, in a special report published on May 1, 1988, investigated fallout exposure in the tri-state region, including some of the nation’s highest readings for radioactivity in milk in the Mandan area and wheat near Crookston, Minnesota

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Fargo was cited as an example of a “hot spot,” an area of unusual fallout, in 1959 congressional hearings, The Forum reported. Radioactive debris in the soil increased more than tenfold from June to July 1957 after rains from fallout clouds.

The detonation of atomic and nuclear bombs at the Nevada Test Site were timed so that winds would carry the fallout away from population centers, including Las Vegas and Los Angeles.

That meant the fallout plumes, carried by winds in the upper atmosphere, often traveled to the north and east — toward the Dakotas and Minnesota.

Maps illustrating the study’s estimated fallout density show high concentration levels reached the western Dakotas, with more diffuse high levels to the east.

Not surprisingly, the highest fallout densities were clustered in the Southwest, including New Mexico, Utah and Nevada, and extended north to Colorado, Idaho and Wyoming, the study found.

Little has been done nationally to determine possible health effects from radioactive fallout from nuclear tests, despite the known link between radiation and cancer. The higher the dose, the greater risk of developing cancer over time.

A study in 1994 by the North Dakota Department of Health, spurred in part by The Forum’s special report, concluded that a “conservative estimate” of between 50 and 100 cases of thyroid cancer could have occurred because of radioactive fallout.

The study also determined three or four cases of childhood leukemia could have resulted from fallout and found North Dakota’s rise in the childhood leukemia death rate from 1963 to 1967 was “consistent with a radiation effect.”

“It’s something that’s been largely forgotten or completely forgotten,” said Dr. Stephen McDonough, who, as chief of the preventive health section, was the study’s lead author.

Although the study found elevated levels for certain cancers associated with radiation, it was not able to prove a causal link, he said.

“You can’t prove it, that the nuclear fallout killed children, but it’s certainly in the realm of possibility,” he said. “I did make a serious effort to look at it. I tried to be thorough.”

North Dakota’s milk had higher levels of radioactive Strontium-90 in milk than other areas of the country from 1955 to 1965. “As a result, North Dakotans were exposed to among the highest amounts of measured dietary Strontium-90 in the United States,” the study found.

A 1966 federal study estimated infants fed milk produced in the Fargo area after the July 1957 fallout episode received a significant dose of radioactive iodine, The Forum reported in 1988. The Fargo levels for one summer were comparable to those accumulated over four years by infants in a Utah fallout town.

Multiple myeloma, a form of cancer that has been associated with radiation, was elevated in Burleigh County, which includes Bismarck, from 1968 to 1987. “This elevated death rate is consistent with the delayed onset of non-leukemia tumors after radiation exposure,” the study said.

The study found that of 22 types of cancer, along with infant mortality, only childhood leukemia, thyroid cancer and multiple myeloma showed possible associations with fallout in North Dakota.

“Despite the heavier than average fallout, North Dakota was, and remains, a healthy place to live,” the study said.

A study in 1987 by the South Dakota Department of Health concluded there were 111 more leukemia deaths than expected, or 10%, from 1950 to 1969. Extrapolating from those additional leukemia deaths, the study calculated there could have been another 1,000 deaths from other cancers due to radioactive fallout plumes.

McDonough said the federal government has failed to adequately study possible cancers and other health effects throughout the United States as a result of its atmospheric nuclear testing program.

“The question is, does the U.S. government owe something to those who got thyroid cancer or whose child died from leukemia during that period,” he said.

Detonating nuclear weapons above ground sent radioactive materials as high as 50 miles into the atmosphere. Large particles fell to the ground near the blast site, but lighter particles and gases migrated into the upper atmosphere, according to the Environmental Protection Agency.

Hundreds of above-ground tests were conducted around the world from 1945 to 1963. Most atmospheric testing ended following an international treaty in 1963, including in the United States and the former Soviet Union, with the last above-ground nuclear test conducted in 1980, according to the EPA.

Fallout typically contains hundreds of radionuclides. Some persist for long periods, such as Cesium-137, which has a half life of about 30 years. Most have short half lives and decay away within minutes or hours. Iodine-131, associated with thyroid cancer, has a half-life of eight days.