SEOUL, South Korea (AP) — South Korean troops captured a North Korean man who crossed the heavily fortified border on Wednesday, the military said.

The Joint Chiefs of Staff said that government agencies will launch a joint investigation to find out why and how the man crossed the mine-strewn border into South Korea.

Earlier Wednesday, the military said South Korea’s surveillance equipment spotted an unidentified person on the eastern section of the land border and launched a search operation. Hours later, it said the military “safely” took custody of the man.

More than 30,000 North Koreans have fled to South Korea in the past two decades, mostly via China, but a small number of North Koreans still occasionally escape through the land border. At the height of Cold War rivalry, both Koreas routinely sent agents and spies to each other’s territory through the border, but no such incident have been reported in recent years.

South Korea's media reported the military boosted its anti-infiltration readiness level in the eastern front-line area on Wednesday, but the Defense Ministry said it couldn’t confirm the report.

North Korea’s state media didn't immediately comment on the man’s border crossing. South Korea said it hasn’t detected any unusual activities by North Korea’s military.

The reported border breach came as South Korea’s government reopened the southern side of an inter-Korean border village to civilian visitors after halting tours for a year due to the spread of African swine fever in the region that forced workers to cull around a half a million pigs. That border village, called Panmunjom, is located on the western part of the land border.

Seoul’s Unification Ministry has said the tours to Panmunjom will begin on a trial basis on Wednesday before officially reopening on Friday.

The two Koreas remain split along the 248-kilometer (155-mile) long border, the world’s most heavily guarded, since the end of the 1950-53 Korean War.

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Associated Press writer Kim Tong-hyung contributed to this report.