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Since the North's last test, in 2009, during President Obama's first months in office, the United States has lost much of its visibility into what a former senior intelligence official says is on the cusp of becoming a "runaway program." Inspectors have been ejected from the country, and new facilities to make nuclear fuel have appeared. And after the North warned last week that it would now conduct a "higher level" test "targeted" at the United States, Kurt M. Campbell, the assistant secretary of state for East Asia, conceded that "we don't know the kind of test that is anticipated."

Now the hope is that an underground blast will answer several mysteries. Can the North Koreans produce a bomb out of uranium — a program they invited a visiting American nuclear scientist to glimpse two years ago — as well as the plutonium bombs that they exploded in 2006 and 2009? Can they make a warhead small enough to fit atop one of the long-range missiles they successfully tested last month?

In short, is it possible that the country that gained a reputation as the Keystone Kops of nuclear nations, setting off nuclear explosions that sputtered and missiles that crashed into the sea, has actually gotten its act together to the point that it now may pose a significant threat?

"It's clear that there is now an expectation that this test could cross a threshold and yield data we haven't had," said Michael Green, a senior director for Asian affairs in the National Security Council under President George W. Bush. "We know a lot about their programs, but not the most important part: how far along are they? And we won't know that until they test."

The test could show, he said, "whether they can build a bomb that can approach Hiroshima or Nagasaki levels, and that would tell us a lot about how far they have proceeded on weaponization."

Previous tests have obviated the question of whether the North is capable of setting off a crude nuclear device. The far bigger question is whether one of the world's poorest, most backward nations can make the kind of leap that Pakistan made in the 1990s, when it tested a bomb and began building an arsenal now estimated to be well in excess of 100 weapons.

The United States has already deployed equipment to measure the future test, including sensitive sniffing devices mounted on reconnaissance planes that may be able to answer the question of whether the North has moved to a new generation of homemade uranium weapons. The Bush administration accused the North of trying to go down that road in 2002, but American intelligence agencies missed a crucial development in recent years, the construction of a large enrichment facility in the heart of the North's main nuclear reactor site.

But the Americans are not the only ones who are focused on the North's progress. So is Iran, which has been struggling with the same uranium technology for years, but has stopped short of conducting a test. "They will certainly be watching," said Joseph R. DeTrani, who was the intelligence community's top North Korea watcher for many years and went on to run the intelligence group created to fight weapons proliferation. "They want to see how it works, and they want to see how North Korea is treated by the rest of the world if they do another test."

The White House has played down the threat from the North and has repeated the mantra that a test would further "isolate" the country, a term that both the Obama and Bush administrations have used, to little effect. But senior American military commanders have noted that the missile that the North tested in December, which went as far as the Philippines and launched a small, light satellite, was a success — a notable change after several missiles fell quickly into the sea.

Similarly, many nuclear experts viewed the North's first two nuclear explosions as laughable flops, if not complete failures.