The announcement increased suspicion that North Korea was involved in what has been described as the worst naval disaster in South Korea's history.

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TOKYO — An “external explosion” probably sank a South Korean naval ship three weeks ago near a disputed sea border with North Korea, a government investigator said Friday in Seoul.

The announcement increased suspicion that North Korea was involved in what has been described as the worst naval disaster in South Korea’s history. At least 38 sailors were killed; eight remain missing. Rescuers found 58 survivors.

North Korea’s possible link to the ship’s sinking has halted U.S. efforts to persuade Kim Jong Il’s government to rejoin six-party nuclear-disarmament talks. It also raises difficult questions about what, if anything, South Korea and the United States might do to retaliate against an unpredictable dictatorship that maintains an artillery arsenal capable of killing many thousands of civilians in Seoul.

The wrecked stern of the 1,200-ton Cheonan was pulled from the Yellow Sea on Thursday, giving South Korean experts their first chance to examine what ripped the ship apart March 26. The ship sank during a routine patrol near the North-South maritime border that has been the scene of three bloody naval skirmishes between the two Koreas.

Military officials in Seoul have speculated the ship was struck by a torpedo or hit a mine. But the governments of South Korea and the United States have taken pains not to accuse the North of involvement, saying they want a thorough investigation. Experts from the United States and other countries are participating in the investigation.

The cautious tone continued Friday, with state investigators saying they needed more time before drawing a conclusion.

Yet there was a slightly harder edge to comments by South Korean Defense Minister Kim Tae-young, who said that once his government determines what caused the explosion, “we will respond in a very clear and firm manner.”

North Korea has not commented publicly on the incident, but its diplomats have told Chinese officials the North was not involved, according to reports in the South Korean news media.

At a briefing in Seoul, South Korea’s investigative team said the sinking of the Cheonan did not appear to have been caused by an onboard accident or by running aground.

“There is a high possibility of an external explosion rather than an internal explosion,” said Yoon Duk-yong, co-head of a state investigation team. “A strong force was applied to the left side of the ship, leaving the hull and iron sheets curved inward. This kind of destruction is caused by an external explosion.”

He said evidence gathered from wreckage showed the ship’s ammunition room, its fuel tank and diesel-engine room were not damaged, and noted there was no indication of an internal fire.

It may take months to determine what sank the warship. Experts have said underwater mines left from the Korean War in the early 1950s still could be in waters where the ship sank. Most speculation, though, has focused on the possibility of a torpedo attack.

“Small North Korean submarines have ventured into South Korean waters in the past,” said Kwon Tae-young of the Korea Research Institute for Strategy, a think tank in Seoul. “But to know the precise cause, scientific research would need to identify torpedo debris.”

Evidence of a North Korean torpedo or mine would not clarify how South Korea or the United States should respond, Kwon said.

“Even if North Korean involvement becomes clear, a military option will be hard to execute,” he said.

North Korea often threatens South Korea and the United States with “all-out war.” The North also has said it has the ability to turn metropolitan Seoul, just 35 miles from the border and with a population of about 22 million, into “a sea of fire.”

That is not an empty threat, according to many military analysts.

North Korea has moved about 70 percent of its military units and up to 80 percent of its artillery to within 60 miles of the border, according to the Strategic Studies Institute, a research arm of the U.S. Army War College.

Special Washington Post correspondent June Lee in Seoul contributed to this report.