China has sent an official request asking Seoul to help retrieve ancient mural paintings plundered a decade ago, saying its investigation found the paintings were in South Korea, the heritage administration said Wednesday.

The Cultural Heritage Administration of Korea said its Chinese counterpart, the State Administration of Cultural Heritage, recently sent a letter saying the mural paintings, stolen from an ancient tomb in the city of Jian in China's eastern Jilin Province in the late 1990s, were believed to have been taken to Korea.

The paintings date back to the Koguryo Kingdom that controlled the northern half of the Korean Peninsula and northeastern China during its reign from 37 B.C. to 668 A.D. Its people left paintings on the walls and ceilings of tombs depicting their daily lives and mythical beliefs. Those rare relics that now belong to North Korea and China have been inscribed on the UNESCO World Heritage list.

China reportedly arrested and executed three Korean-Chinese citizens in 2003 on charges of stealing several Koguryo mural paintings in Jian and selling them to a South Korean surnamed Lee.

In the letter, the Chinese authorities said the convicts consistently said that the mural paintings had been passed on to South Korea. The letter did not identify the route of their illegal transfer nor present any direct evidence. It only cited an investigative program recently aired by South Korean broadcaster MBC, which suggested possible connections between the plunderers and the Korean Antiques' Association, a private antique dealer in Seoul.

"The letter mainly states that China asks for the South Korean government's cooperation in its efforts to retrieve the cultural properties," the Korean heritage agency said in a statement.

The agency said the paintings had not been located. It is considering asking the police and other authorities to start an official search.

The legacy of the Koguryo Kingdom is a sensitive issue between Korea and China, which also lays claim to the kingdom's historic roots.