China sent an emergency alert to Washington, D.C., through the U.S. Embassy in Beijing at which time President George W. Bush was told by National Security Advisor Stephen Hadley "shortly after" 10 p.m. (UTC-5) that a test was imminent.

In late 2002 and early 2003, North Korea began to take steps to eject International Atomic Energy Agency inspectors while re-routing spent fuel rods for plutonium reprocessing for weapons purposes.

As late as the end of 2003, North Korea claimed that it would freeze its nuclear program in exchange for American concessions – in particular a non-aggression treaty – but a final agreement was not reached and talks continued to be cancelled or fall through.

In early 2004 former Los Alamos National Laboratory director Siegfried S. Hecker, as part of an unofficial U.S. delegation, was allowed to inspect North Korea's plutonium production facilities.

Hecker later testified before the United States Congress that while North Korea seems to have successfully extracted plutonium from the spent fuel rods, he saw no evidence at the time that they had actually produced a workable weapon.

[13] In 2007, the former senior scientist of Pakistan, Dr. Abdul Qadeer Khan claimed that North Korea's nuclear program was well advanced before his visit in 1993 with Benazir Bhutto, former prime minister.

[14][dubious – discuss] In September 2004, though, North Korean officials announced they had successfully processed Yongbyon plutonium into a workable nuclear deterrent.

The Korean Central News Agency, the state's news agency, issued the following statement:[16] The field of scientific research in the DPRK successfully conducted an underground nuclear test under secure conditions on October 9, Juche 95 (2006), at a stirring time when all the people of the country are making a great leap forward in the building of a great, prosperous, powerful socialist nation.

"[20] On the day of the test, the North Korean Foreign Ministry stated that "if the U.S. keeps pestering us and increases pressure, we will regard it as a declaration of war and will take a series of physical corresponding measures".

The agreement was struck in a day of unpublicized discussions between the senior envoys from the United States, China and North Korea at a government guesthouse in Beijing.

[9][25] Radioactive isotopes of the element xenon are produced by the atom splitting that takes place in nuclear explosions and readily seeps out even from underground tests.

An early report by the Institute of Geoscience and Mineral Resources of South Korea said the blast was equivalent to an earthquake registering 3.58 on the Richter scale,[28] which corresponds to the explosion of 100 tons of TNT.

According to Jane's Defence Weekly, "initial and unconfirmed South Korean reports indicate that the test was a fission device with a yield of 0.55 kT ...

According to the Washington Times anonymous U.S. intelligence sources speculated there "was a seismic event that registered about 4 on the Richter scale, but it still isn't clear if it was a nuclear test.

[37] If the North Korean device was significantly short of its predicted yield, it could be classified as a "fizzle" indicating that some aspect of the nuclear weapon design or material production did not function correctly.

In a fizzle the warhead blows itself apart too fast for the nuclear reactions to generate a large amount of energy, or fails to form a supercritical mass for some other reason.

[39] The explosion was also recorded worldwide by the global monitoring system operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO).

Two weeks after the detonation, a CTBTO radionuclide monitoring station in northern Canada detected traces of the radioactive noble gas xenon in the air.

[40] According to initial reports from South Korean government sources, the test was carried out at a mountain in Musadan-ri in Hwadae-kun, near the city of Kilchu, in North Hamgyŏng province on the northeast coast.

[28] The United States Geological Survey reported that a seismic event occurred at 01:35:28 UTC (10:35:28 am local time, UTC+9) on October 9, 2006, and measured 4.3 on the Richter scale.

[43] International condemnation of the tests by governments has been nearly unanimous, including from North Korea's close ally and benefactor, the People's Republic of China.