SEOUL, South Korea (AP) — Twenty-one North Koreans were found in a boat drifting in South Korean waters this week, the largest such arrival in nine months, an official said Saturday.

It wasn’t immediately clear how or why the North Koreans, who remain in custody, ended up in South Korean territory. South Korea takes in North Koreans seeking to defect, but puts them through rigorous questioning. Those who ask to return home are repatriated.

The boat was found drifting near the western island of Socheong, according to a South Korean coast guard official in the port city of Incheon. Socheong lies just south of the Koreas’ disputed maritime border.

The official said he could not provide more details because authorities were still questioning the North Koreans. He refused to be identified, saying he was not allowed to speak to the media.

South Korea’s National Intelligence Service declined to comment, and neither Unification Ministry nor Joint Chiefs of Staff officials answered phone calls Saturday.

The arrival is the largest of its kind since February, when 31 North Koreans floated south on a fishing boat. Seoul sent 27 back at their request and let four stay in the South.

Citizens of the two Koreas cannot travel freely between their countries, which have been divided by a heavily fortified Demilitarized Zone since the Korean War ended in a truce in 1953.

However, North Korean boats occasionally drift south of the Koreas’ Yellow Sea maritime border, an area prized by fishermen from both Koreas. Some ask to return home, but some choose to defect.

More than 21,000 North Koreans have defected to South Korea since the war, most through China and Southeast Asia, according to the South Korean government.