Just as relations between South Korea and the Democratic People's Republic of Korea (DPRK) reached their lowest ebb in many years, a mysterious naval tragedy near a disputed maritime border has left South Korea grappling with a question over possible involvement of Pyongyang and cast more shadow over the peninsula.

A man grieves during a memorial service for the deceased sailors from the lost ROK naval ship Cheonan on Monday. [Ahn Young-joon / Associated Press]

Investigators looking into the sinking of a 1,200-ton corvette a month ago announced Sunday that a powerful external explosion at a close range, possibly caused by a torpedo or sea mine detonation, was the most likely cause of the incident.

"We have concluded that a non-contact explosion is the likely cause, considering that there is no soot on the ship's body or traces of melted material under the heat," said Yoon Duk-yong, the civilian head of the 43-member joint investigation team, who ruled out other possible causes such as metal fatigue of the ship or a collision with reefs.

South Korean defense chief Kim Tae-young also weighed in prior to the announcement by the investigators, publicly pointing a finger at possible torpedo attacks as a likely culprit but stopping short of accusing Pyongyang.

"A bubble jet caused by a heavy torpedo is believed to be the most likely cause of the incident, but we are still considering other possibilities," he said, referring to a strong shockwave and gas bubble made when a torpedo or a mine detonates under a ship.

The naval warship Cheonan with 104 crew members aboard sank on March 26 into tense waters off the west coast of the Korean peninsula, a scene of two deadly naval skirmishes in the past decade, after an unexplained blast split the ship in half.

With the authorities seen scrambling unsuccessfully to determine the cause, the sheer magnitude of the disaster, said to be one of the worst peacetime tragedies, quickly ignited speculation that the DPRK might have been behind the sinking.

Twenty-two days after the disaster, the DPRK's official Korean Central News Agency (KCNA) issued an article, expressing its wrath over the South Korean speculation.