In July 1955, researchers from North Korea's Academy of Sciences traveled to Moscow to attend a conference on nuclear energy. Shortly thereafter, the Kim Il Sung regime began sending specialists to a new institute in Dubna, a city devoted to Soviet nuclear physics. Their mission: learn the basics of splitting the atom.

From that seed, a bomb may have grown. On Oct. 9 – more than a half century after that first delegation left Pyongyang – North Korea conducted a nuclear test in the northeastern region of Kilju.

The test was small, and possibly a failure. But US experts say that given the length of time the North Koreans have been at work, and the apparent importance of nuclear weapons to the country's leadership, they may now continue to test until they get it right.

They might not have labored five decades just to produce a scientific demonstration. At this point, they could be perfecting something else: a deliverable nuclear arsenal.

"My own view is that they are building a smaller weapon in line with a device that could fit on a ballistic missile," says David Albright, a physicist and president of the Institute for Science and International Security (ISIS) in Washington.

Possibility of a second test

At the time of writing, North Korea had not conducted a second nuclear explosion. But US satellite imagery reportedly has revealed increased activity around several possible test sites.

"We're concerned about further action by the North Koreans, but further action by the North Koreans will only deepen its isolation, which is pretty deep right now," said Secretary of State Condoleezza Rice on Wednesday in Tokyo.

North Korean officials, for their part, did not rule out a second test. In a rare interview with a Western news organization, Ri Gun, director-general of the Foreign Ministry's American affairs bureau, told ABC News that another test would be "natural."

That is just what worries nuclear experts in the United States.

According to a statement issued by the office of John Negroponte, US director of national intelligence, the explosive yield of the Oct. 9 test was "less than a kiloton." It is possible that this relatively low yield was intentional, the result of a proof-of-principle design using minimal fissile material. But it is much more likely to indicate a test fizzle, say experts.

"They will want to test until they get it right," said Michael Swaine, a senior associate in the China program at the Carnegie Endowment for International Peace in Washington, at an Oct. 12 seminar on the North Korean situation.

Such misfires aren't necessarily symbols of incompetence. Almost all declared nuclear powers have had them. In October 1951, a US bomb test named Buster Able failed not once, but twice. The Soviets had a misfire in 1954. In May 1957, Britain's first attempt at exploding a hydrogen bomb produced only a quarter of the expected yield.

In the case of North Korea, whatever went wrong, it probably was not due to haste. Satellite photos of the suspected test site released Oct. 13 by the Natural Resources Defense Council (NRDC) show that neither buildings nor roads changed much from late 2005 to October 2006.

"The North Koreans probably did not rush the test," wrote physicist Matthew McKinzie, an NRDC consultant, in an analysis of the photos.

Indeed, the photos show that excavation activity for the test may have been carried out in February 2005.

Instead, the misfire may indicate that the device was a somewhat sophisticated plutonium-fueled bomb, according to the NRDC.

Via atmospheric sampling, US intelligence has determined that plutonium was used in the North Korean test, according to news reports. For such a weapon to explode efficiently, the plutonium core must be "squeezed" by conventional explosives in a highly precise manner.

"It is possible that the detonators igniting the high explosive that compresses the plutonium did not fire simultaneously and thus only produced a partial yield," concludes the NRDC analysis.

Why alarm rose in the 1980s

Though North Korea's interest in nuclear science was well known in the West, it was not until the mid-1980s that US intelligence began to become alarmed about Pyongyang's intentions.

Until 1984, North Korea was not viewed as a serious nuclear proliferation threat. The Central Intelligence Agency had thought the country's nuclear program to be rudimentary, and not capable of producing the fissile material needed for weapons, according to "Spying on the Bomb," a recent book about US nuclear intelligence by Jeffrey Richelson.

But in 1986, a top-secret CIA reassessment titled "North Korea: Potential for Nuclear Weapons Development," drawing on unspecified new information, held that Pyongyang might indeed be capable of a making a bomb. The CIA speculated that North Korea's motives might be forcing political concessions from South Korea, hedging against other Asian powers going nuclear, or deterring a US nuclear response in case of hostilities on the Korean peninsula, according to Mr. Richelson, a senior fellow with the National Security Archive.

US concerns then began to mount by the year. In 1987, North Korea received from a West German company a small annealing furnace capable of helping to produce centrifuge rotors for uranium enrichment. An inquiry by German intelligence concluded in 1990 that North Korea had possibly obtained crucial information on uranium melting from Pakistan in the late 1980s.

Media reports indicate that North Korea allegedly did business with the notorious Pakistani proliferator A.Q. Khan throughout this period. Dr. Khan might have provided everything from rotor designs to a Chinese design for a basic atomic device, according to a Congressional Research Service study on North Korea's nuclear trade.

When North Korea withdrew from the Non-Proliferation Treaty in 2003, it apparently moved quickly to process weapons-grade plutonium from power reactors that had been subject to international inspections. It is that fuel that probably powered this month's test.

Further tests might increase North Korea's confidence in its weapons design. Pyongyang might also decide that another explosion would increase the deterrent value of its arsenal.

However, more tests might also shrink North Korea's small plutonium stockpile. That makes it all the more likely that North Korea's scientists are perfecting a deliverable weapons, says Mr. Albright of ISIS.

"If you have a chance to test, you want to test something more in line with what you want to deploy," he says.