SEOUL, South Korea (AP) — South Korea's president Monday bemoaned the "precious deaths" of dozens of sailors killed when an explosion destroyed their warship, as suspicion increasingly fell on North Korea in the blast.

Makeshift altars have been set up across South Korea for a five-day mourning period that opened Sunday to honor victims aboard the 1,200-ton Cheonan when it split into two in an unexplained explosion on March 26. President Lee Myung-bak visited one of the shrines at a plaza in Seoul on Monday to pay his respects.

"The Republic of Korea will never forget your precious deaths," Lee wrote at a condolence book at the mourning station, according to South Korean media pool reports.

Last week, a tearful Lee said in a nationally televised speech that South Korea will respond "resolutely and unwaveringly" against anyone responsible for the blast.

On Sunday, Defense Minister Kim Tae-young said that an underwater explosion appeared to have ripped apart the vessel, and a torpedo blast seemed the most likely cause. Investigators who examined salvaged wreckage separately announced Sunday that a close-range, external explosion likely sank it.

"Basically, I think the bubble jet effect caused by a heavy torpedo is the most likely" cause, Kim told reporters Sunday. The bubble jet effect refers to the rapidly expanding bubble an underwater blast creates and the subsequent destructive column of water unleashed.

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

Soon after the disaster, Kim told lawmakers that a North Korean torpedo was one of the likely scenarios, but the government has been careful not to blame the North outright, and Pyongyang has denied its involvement.

As investigations have pointed to an external explosion as the cause of the sinking, however, suspicion of the North has grown, given the country's history of provocation and attacks on the South.

The Cheonan was on a routine patrol before the explosion sank it in one of South Korea's worst naval disasters. Forty bodies have been recovered so far, but six crew members are still unaccounted for and are presumed dead.

The site of the sinking is near where the rival Koreas fought three times since 1999, most recently a November clash that left one North Korean soldier dead and three others wounded. The two Koreas are still technically at war because their 1950-53 Korean War ended in a truce, not a peace treaty.

Also Sunday, investigators said a preliminary investigation of the front part of the 1,200-ton ship — retrieved the day before — pointed to an external explosion.

Chief investigator Yoon Duk-yong told reporters that an inspection of the hull pointed to an underwater explosion. He appeared to support the bubble jet effect theory, saying, "It is highly likely that a non-contact explosion was the case rather than a contact explosion."

But he, too, said it was too early to determine what caused the explosion.