SEOUL, South Korea (AP) — A Seoul-based activist alleged Tuesday that a squad of North Korean soldiers was behind last month’s deadly sinking of a South Korean frigate.

The activist cited a North Korean military officer claiming knowledge of the plot.

The unidentified officer said a North Korean semi-submersible vessel carrying 13 crewmembers fired a torpedo at the Cheonan, according to Choi Sung-yong, who said he had spoken to the officer by telephone several times in recent days.

The claim could not be verified, and the Joint Chiefs of Staff in Seoul said it could not confirm the allegation.

An explosion split the 1,200-ton Cheonan in two on March 26 while the ship was on a routine patrolling mission in the western waters near the tense maritime border with North Korea. Fifty-eight sailors were rescued, but at least 38 died and eight are missing.

The military officer told Choi the soldiers are being hailed as heroes in North Korea, Choi told The Associated Press.

Seoul has not openly blamed Pyongyang for the sinking of the Cheonan, one of South Korea’s worst naval disasters. North Korea has denied involvement.

However, communist North Korea has a record of attacking the South, its wartime rival, and suspicion of North Korean involvement is growing in Seoul. South Korean officials said they were investigating the possibility that a North Korean mine or torpedo struck the warship.

The two Koreas have never signed a peace treaty since the 1950-53 Korean War ended in a truce.

Their militaries have clashed three times in the Yellow Sea, most recently in November. A North Korean sailor was killed and three others wounded in that battle, South Korean officials said.

Choi, whose father was abducted to North Korea decades ago, heads an association of relatives of abductees. Now an activist, he has been involved in bringing South Korean abductees and prisoners of war out of North Korea, and claims to have regular contact with several North Koreans, including the military officer.

He said the North Korean officer told him leader Kim Jong Il ordered troops to retaliate during a visit to the western North Korean port of Nampo right after the November defeat.

Choi declined to identify the North Korean officer. His claim first was reported in Seoul’s Chosun Ilbo newspaper.

A November dispatch in the official Korean Central News Agency said Kim inspected a navy command but did not say where.

Since then, two top North Korean military leaders, Vice Marshal Kim Il Chol and Gen. U Tong Chuk, have made frequent visits to Nampo to map out military operations, while a naval commander, Jong Myong Do, stayed to monitor the operation, the officer told Choi.

KCNA reported on April 14 that U and Jong were promoted last week as part of celebrations on the 98th anniversary of North Korea founder Kim Il Sung’s birth, a major North Korean holiday.

Speculation of North Korean involvement has mounted since the probe’s chief investigator said Friday that an explosion appeared to have come from outside the ship. A torpedo or mine are among suspected culprits.

South Korean President Lee Myung-bak, making a tearful address to the nation, vowed Monday to deal “resolutely and unwaveringly” with the outcome of the investigation.

Defense Minister Kim Tae-young has said there is no definitive evidence yet indicating North Korean involvement.

He told lawmakers Monday that sailors testified that the ship’s sonar did not detect any signs of an approaching torpedo.

Investigators have found no shrapnel from a torpedo or a naval mine, the Joint Chiefs of Staff said.

• Kwang-Tae Kim