After an initial examination of the stern of the Navy corvette Cheonan on Thursday, the military tentatively concluded that the corvette had not been hit directly by a torpedo but broke in two due to a bubble jet created by an underwater explosion. The stern was raised 20 days after the corvette sank in the West Sea on March 26.

The military also speculates that a torpedo attack was a much more likely cause of the explosion than a mine.

"In an initial examination of the Cheonan's stern, South Korean and U.S. investigators found no traces showing that the hull had been hit directly by a torpedo," a senior source at the Defense Ministry said. "Instead, they found traces proving that a powerful explosion caused possibly by a torpedo had occurred underwater. The explosion created a bubble jet that eventually generated an enormous shock wave and caused the ship to break in two."