A North Korean fishing boat carrying three passengers crossed the marine border into South Korea in the East Sea late Saturday, Seoul’s military said Sunday.

The boat has been towed to a South Korean military port for investigation.

According to the Joint Chiefs of Staff, the wooden boat was first spotted about 5.5 kilometers above the Northern Limit Line, the de facto maritime border, at around 10:15 p.m. on Saturday. It started moving down in a straight line to cross the NLL at 11:21 p.m.

The South Korean Navy, which was keeping watch, immediately deployed a high-speed boat, a rigid-hulled inflatable boat and a patrol frigate to the scene.

“The North Korean crew said they lost their way and crossed the border by mistake,” a JCS official explained.

When South Korean Navy officers checked the boat and asked the three crew members whether they had come with the purpose of defecting, they reportedly denied the notion.

The three North Koreans were taken to a naval port in Yangyang, Gangwon Province, for investigation. The 10-meter wooden boat was also towed.

The JCS said there was an abundance of fishing gear on the boat, with squid in the storage cabin. A white towel was tied to the mast.

Not all North Korean fishing boats are brought to the South for investigation. But the fishing boat on Saturday was towed as there were some peculiar points that need to be clarified, the JCS official explained.

While the North Korean crew said they crossed into South Korean seas by mistake, they entered in a straight line, the JCS said. The tag on the boat also showed the boat to be operated by the North Korean military, though further investigation is needed to confirm whether the fishermen are from the North Korean military.

The boat was the only vessel spotted around the NLL area at the time.

The South Korean military appears to have taken swift action on the North Korean boat, as it came under fire for letting a fishing boat from the North reach a South Korean port city undetected in June.

At the time, the North Korean boat carrying four sailed through the East Sea and reached Samcheok Port in Gangwon Province. Two crew members even came up on land to speak with South Korean civilians.

In the joint government investigation, two expressed their desire to stay in the South, and the other two were returned to North Korea.

About 380 North Korean fishing boats were expelled for illegally fishing below the NLL from May 31 to July 14, according to the Defense Ministry. The number appears to have surged from last year’s 40, as shoals of squid have increased around the border line this year.

By Jo He-rim (herim@heraldcorp.com)