South Korea’s military said Sunday that an unidentified person crossed the heavily fortified border into North Korea

South Korea had earlier spotted the person with surveillance equipment at the eastern portion of the border and sent troops to capture him or her on Saturday night. But the troops failed to find the person and the surveillance equipment detected the person crossing over the border, Joint Chiefs of Staff officers said.

South Korea sent a message to North Korea on Sunday morning to ensure the safety of the person, but the North hasn’t responded, the officers said requesting anonymity citing department rules.

In September 2020, North Korea fatally shot a South Korean fisheries official found floating in its waters in line with what Seoul called strict anti-virus rules that involve shooting anyone illegally crossing the border. Earlier in 2020, North Korean leader Kim Jong Un placed a border city under total lockdown after a North Korean defector with COVID-19-like symptoms sneaked back home. The fate of that defector, who had lived in South Korea, is not known.

On Saturday, North Korea announced it had decided to place top priority on strict virus restrictions at a high-profile ruling party meeting last week.

The two Koreas are split along the world’s most heavily armed border, called the Demilitarized Zone. An estimated 2 million mines are peppered inside and near the 248-kilometer (155-mile) -long, 4-kilometer (2.5-mile) -wide DMZ, which is also guarded by barbed wire fences, tank traps and combat troops on both sides.

Defecting via the DMZ is subsequently rare. At the height of their Cold War rivalry, both Koreas sent agents and spies to each other’s territory through the DMZ, but no such incidents have been reported in recent years.

About 34,000 North Koreans have defected to South Korea since the late 1990s to avoid poverty or political oppression, but a vast majority of them have come via China and Southeast Asian countries. Some South Koreans have tried to defect to the impoverished, authoritarian North, but it is rare.