Investigation on 13 September 2010. After raising the ship, South Korea and the United States formed a joint investigative team to discover the cause of the sinking. Later South Korea announced that it intended to form an international group to investigate the sinking including Canada, Britain, Sweden and Australia. On 16 April 2010, Yoon Duk-yong, co-chairman of the investigation team, said "In an initial examination of
Cheonans stern, South Korean and U.S. investigators found no traces showing that the hull had been hit directly by a torpedo. Instead, we've found traces proving that a powerful explosion caused possibly by a torpedo had occurred underwater. The explosion might have created a
bubble jet that eventually generated an enormous shock wave and caused the ship to break in two." Traces of an explosive chemical substance used in torpedoes,
RDX, were later found in May 2010.
The Washington Post reported on 19 May 2010, that a team of investigators from Sweden, Australia, Britain, and the United States had concluded that a North Korean torpedo sank the ship. The team found that the torpedo used was identical to a North Korean torpedo previously captured by South Korea. On 25 April 2010, the investigative team announced that the cause of the sinking was a non-contact underwater explosion. On 7 May 2010, a government official said that a team of South Korean civilian and military experts had found traces of
RDX, a high explosive more powerful than
TNT and used in torpedoes. On 19 May 2010, the discovery of a fragment of metal containing a serial number similar to one on a North Korean torpedo salvaged by South Korea in 2003 was announced. In their summary for the
United Nations Security Council, the investigation group was described as the "Joint Civilian-Military Investigation Group of the Republic of Korea with the participation of international experts from Australia, Sweden, the United Kingdom and the United States, and the Multinational Combined Intelligence Task Force, comprising the Republic of Korea, Australia, Canada, the United Kingdom and the United States," which consisted of "25 experts from 10 top Korean expert agencies, 22 military experts, 3 experts recommended by the National Assembly, and 24 foreign experts constituting 4 support teams". in which they concluded that the sinking of the warship was the result of a North Korean torpedo attack, commenting that "The evidence points overwhelmingly to the conclusion that the torpedo was fired by a North Korean submarine." The inquiry also alleged that a group of small submarines, escorted by a support ship, departed from a North Korean naval base a few days before the sinking. The specific weapon used was alleged to be a North Korean manufactured
CHT-02D torpedo, of which substantial parts were recovered. The torpedo utilizes acoustic/wake homing and passive acoustic tracking methods. Such detailed information on the North Korean submarine movements, and attack position, was not in the official summary or final report. The torpedo parts recovered at the site of the explosion by a dredging ship on 15 May which include the 5×5 bladed
contra-rotating propellers, propulsion motor and a steering section, was claimed to match the schematics of the CHT-02D torpedo included in introductory brochures provided to foreign countries by North Korea for export purposes. An incorrect, though similar, torpedo schematic had by mistake been shown at the televised RIG briefing for comparison with the recovered parts. The correct schematic has never been made public. At the end of the propulsion section, markings in
Korean script reading "1번" (
romanized: , translated: no. 1) were discovered. Some critics have pointed out that these markings are inconsistent with North Korean practices, as the character "번" is not commonly used there, with "호" (
romanized: ) being used in its place instead. This is corroborated by the fact that another North Korean torpedo obtained seven years earlier was indeed marked using the "호" character. Others have refuted that a yet-different captured North Korean torpedo
did use the "번" character, and Lee Kwang Soo, a former North Korean submarine
helmsman living in the South Korea since his capture during the
1996 Gangneung submarine infiltration incident, has also confirmed that the "번" character is "a normal thing in North Korea" and that it is used "for the repair of parts". Russian and Chinese torpedoes are marked in their respective languages. The full report had not been released to the public at this time, though the South Korean legislature was provided with a five-page synopsis of the report. It concluded that
Cheonan had been sunk due to a torpedo explosion, which, while not having contacted the ship, exploded several meters from the hull of the ship and caused a shockwave and bubble effect of sufficient strength to severely damage and sink the ship. , 23 March 2011
South Korean opinions According to a survey conducted by Seoul National University's Institute for Peace and Unification Studies, less than one third of South Koreans trust the findings of the multinational panel. A later survey by the JoongAng Ilbo newspaper in 2011 found that 68 percent of South Koreans trusted the government's report that
Cheonan was sunk by a North Korean submersible. Lee Jung Hee, a lawmaker with the opposition
Democratic Labor Party, was sued for defamation by seven people at South Korea's Joint Chiefs of Staff. Lee said during a speech in the national assembly that while the Defense Ministry had said there was no feed from a thermal observation device showing the moment the warship's stern and bow split apart, such a video did exist. Prosecutors then questioned Shin Sang-cheol, who served on the panel that investigated the incident and also runs Seoprise, over his assertion that
Cheonan sank in an accident and that the evidence linking the North to the torpedo was tampered with. The Defense Ministry asked the National Assembly to eject Shin from the panel for "arousing public mistrust." Shin stated that he doubted the official conclusion on the sinking, saying that when he looked at the dead sailors' bodies, they bore no signs of an explosion. Shin wrote a letter addressed to US Secretary of State Hillary Clinton showing the evidence for his contention that the ship ran aground and then collided with another vessel.
Russian Navy experts assessment inside one of screw hub holes Near the end of May a team of
Russian Navy submarine and torpedo experts visited South Korea to conduct an assessment of the South Korean led investigation. The team returned to Russia with samples for further physical-chemical analysis. No official statement on the assessment has been made. It was claimed that the assessment concluded
Cheonan was not sunk by a North Korean bubble jet torpedo, but did not come to any firm conclusion about the cause of the sinking. A
Indian Newspaper
The Hindu quoted a Russian Navy source stating that "after examining the available evidence and the ship wreckage Russian experts came to the conclusion that a number of arguments produced by the international investigation in favour of the DPRK's involvement in the corvette sinking were not weighty enough". On 27 July 2010,
The Hankyoreh published what it claimed was a detailed summary of the Russian Navy expert team's analysis. According to
The Hankyoreh, the Russian investigators concluded that
Cheonan touched the sea floor and damaged one of its propellers prior to a non-contact explosion, possibly caused by setting off a mine while the ship was trying to maneuver into deeper water. Visual examination of the torpedo parts South Korea found purportedly indicated that it had been in the water for more than 6 months. On the following day South Korean officials responded with "a full-scale refutation". On 3 August 2010 Russian UN ambassador
Vitaly Churkin stated that his country's investigative report's conclusions into the sinking would not be made public. Withholding the investigation results was seen as insensitive by South Korea, but avoided political damage to a number of states.
Chinese statements During talks between the American and Chinese governments in late May 2010, Chinese officials were reported by
Yoichi Shimatsu, a commentator for the Chinese state-run
CCTV-9, to have stated that the sinking of
Cheonan had been as a result of an American
rising mine, which was moored to the seabed and propels itself into a ship detected by sound or magnetics, planted during anti-submarine exercises that were conducted by the South Korean and US navies shortly before the sinking. To back up their statements, the Chinese said that North Korean submarines such as the one believed to have sunk
Cheonan were incapable of moving undetected within South Korean waters, and a rising mine would have damaged the ship by splitting the hull, as was done to
Cheonan, rather than simply holing the vessel as a conventional torpedo does. A conventional torpedo traveling at would also be completely destroyed upon impact, which was stated to contradict the torpedo parts found later.
Other international research A separate investigation conducted by scientists at the
University of Manitoba yielded results that conflict with the official investigation's findings. According to the leader of the investigation, residue on the hull of the ship that was claimed to have been
aluminum oxide, which is a byproduct of explosions such as that of a torpedo, had a far higher ratio of oxygen to aluminum, leading the researchers to conclude that "we cannot say that the substance adhering to the
Cheonan was the explosion byproduct of aluminum oxide." The South Korean Ministry of Defense issued a rebuttal to the findings, saying, "The detonation of explosives containing aluminum occurs within hundreds of thousandths of a second under high temperatures of more than 3,000 degrees Celsius and high pressures of more than 200,000 atm, and most of it becomes noncrystalline aluminum oxide." The article also notes the rebuttal of those disagreements by analysts and government officials, with one analyst arguing that the sinking was "consistent with North Korea's behaviour in the past." In 2014, an academic paper was published analysing the spectra of the seismic signals and hydroacoustic reverberation waves generated. The paper found it doubtful that the vibrations of the water column were caused by an underwater explosion, instead finding that the recorded seismic spectra were consistent with the natural vibration frequencies of a large submarine with a length of around . This raised the possibility that the sinking was caused by a collision with a large submarine, rather than an explosion. ==Reaction==