By Jung Sung-ki
Staff reporter

Despite hard evidence provided by investigators looking into the cause of the March 26 sinking of the Cheonan, questions linger about the cause of the incident.

The multinational investigation team said a detonation of a heavy torpedo with a net explosive weight of 200 to 300 kilograms tore the Navy ship apart.

It said two North Korean submarines, one 300-ton Sango class and the other 130-ton Yeono class, were involved in the attack. Under the cover of the Sango class, the midget Yeono class submarine approached the Cheonan and launched the CHT-02D torpedo manufactured by North Korea, it said.

The CHT-02D uses acoustic/wake homing and passive acoustic tracking methods, the team said. Acoustic homing torpedoes track and target the engine noise from a ship.

But some experts raised the question if a midget submarine could have a system to carry and control such a precision-homing heavy torpedo.

``Sango class submarines are known to be used by North Korean commandos in infiltrating areas or laying mines, but they apparently do not have an advanced system to guide homing weapons," an expert at a missile manufacturer told The Korea Times on condition of anonymity. "If a smaller class submarine was involved, there is a bigger question mark."

Investigators said North Korea's navy possesses a fleet of 70 submarines ― 20 1,800-ton Romeo class submarines, 40 Sango class and 10 midget subs.

"Given the findings combined with the operational environment in the vicinity of the site of the incident, we assess that a small submarine is believed to be the weapon system used in the attack," Rear Adm. Moon Byung-ok, spokesman for the Joint Military-Civilian Investigation Group, told reporters. "We confirmed that two submarines left their base two or three days prior to the attack and returned to the port two or three days after the assault."

Moon's remarks also raised a question about the credibility of South Korean and U.S. authorities. Earlier, South Korean and U.S. military authorities confirmed several times that there had been no sign of North Korean infiltration in the West Sea.

In addition, Moon's team reversed its position on whether or not there was a column of water following an air bubble effect.

Earlier, the team said there were no sailors who had witnessed a column of water. But during Thursday's briefing session, the team said a soldier onshore at Baengnyeong Island witnessed "an approximately 100-meter-high pillar of white," adding that the phenomenon was consistent with a shockwave and bubble effect.

The JIG displayed fragments of a torpedo propeller during Thursday's press conference, citing it as critical evidence. It said the torpedo parts was recovered by fishing vessels May 15, and the debris, including 5x5 bladed contra-rotating propellers, a propulsion motor and steering section, perfectly match the schematics of the CHT-02D torpedo.

But it seemed that the collected parts had been corroding at least for several months.

Yoon Duk-yong, co-head of the investigative group, denied the suspicion.

"The corrosion status of the fragments and wreckage of the Cheonan is almost identical," Yoon said.

Investigators claim the Korean word written on the driving shaft of the propeller parts was same as that seen on a North Korean torpedo discovered by the South off the west coast seven years ago.

"The word is not inscribed on the part but written on it," an analyst said. "I personally trust the investigators tried their best to prove the link between North Korea and the Cheonan sinking, but the lettering issue is dubious."