South Korean President Lee Myung-bak on April 26 bemoaned the "precious deaths" of dozens of sailors killed when an explosion destroyed their warship, as suspicion increasingly fell on a North Korean torpedo blast in the disaster.

Makeshift altars were set up across South Korea for a five-day mourning period that opened April 25 to honor the 46 victims aboard the Cheonan. Lee visited one of the shrines at a plaza in Seoul to pay his respects.

Defense Minister Kim Tae-young said April 25 that an underwater explosion appeared to have ripped apart the 1,200-ton Cheonan on March 26, and that a torpedo blast seemed the most likely cause. Investigators who examined salvaged wreckage separately announced that a close-range, external explosion likely sank it.

The defense minister told reporters that a likely cause of the disaster was the "bubble jet effect," which refers to the rapidly expanding bubble an underwater torpedo blast can create and the subsequent destructive column of water unleashed.

"Basically, I think the bubble jet effect caused by a heavy torpedo is the most likely" cause, Kim said.

Kim, however, did not speculate on who may have fired the weapon, and said an investigation was continuing and that it's too early to determine the cause.

The government has been careful not to blame the North outright and Pyongyang has denied its involvement, but suspicion has focused on the North given the country's history of provocation and attacks on the South.

AP