Tensions are mounting on the Korean peninsula as the two Koreas exchanged warnings over a South Korean navy ship that sank last March, killing 46 sailors near the disputed western sea border.
A multinational team of experts in Seoul concluded Wednesday that it was a North Korean torpedo that split the vessel in half after an explosion.
In a two-hour live nationally televised press conference, the civilian-military investigative team offered computer simulations, providing what it said was scientific proof of the origins of the attack. The report said a North Korean submarine had fired a torpedo, causing a massive underwater blast.
The smoking gun, according to the investigators, was a propeller that powered the torpedo, which a pair of South Korean fishing boats uncovered at the bottom of the ocean. Its form and size "perfectly matched" the blueprint of torpedoes that Pyongyang, the seat of the North Korean government, had used in its catalogs when trying to sell the weapons abroad, said the chief investigator Yoon Duk-yong.
Also on a fragment of the propeller was a serial number, handwritten as "number 1" in Korean.
The investigative team showed a picture of a stray North Korean torpedo that South Korea had obtained seven years ago, which also had a handwritten "number 4" on it. "There is no other plausible explanation," said the chief investigator. "The evidence overwhelmingly points to the conclusion that it was a torpedo fired by North Korea."
When asked if the team was confident that the recovered fragments of the torpedo were from the same torpedo that had damaged the sunken ship, the investigators explained that the chemical analysis showed traces of explosives that were identical to the ones collected from the vessel.
Pyongyang stood ready to rebut the report. A half an hour into the live press conference, the North Korean National Defense Commission announced through its state-run Central News Agency that the entire report was a fabrication, and that it would send its own team of investigators to South Korea, threatening all-out war.
South Korean President Lee Myung-bak vowed "stern action" against the North and emphasized that the international community would work together on the best way to handle this provocation.
Seoul, Washington and Tokyo have considered options that could include sanctions imposed through the United Nations Security Council, or additional U.S. penalties.
The White House called the attack an unacceptable "act of aggression," and Secretary of State Hillary Clinton planned to visit Seoul during the last leg of her Asia trip next week.
Japanese Prime Minister Yukio Hatoyama pledged his support for international action, calling North Korea's actions "inexcusable" and "hard to forgive."
China, North Korea's last remaining ally, stopped short of offering support. Its foreign ministry said it had "noted" the report but would make its own evaluation. Its spokesman called on both North and South Korea to avoid any escalation in tensions and to "remain calm and show restraint."
ABC News' Chito Romana contributed to the reporting of this story from Beijing.