An American service member willfully crossed the border between South and North Korea and is now in the custody of the north, according to U.S. military officials.

The soldier, assigned to U.S. Forces Korea, was on a Joint Security Area tour and entered North Korea without authorization on Tuesday. The Pentagon has not named the soldier yet, waiting until it is able to notify his next of kin, but the person was identified as 23-year-old Pvt. Travis King, a U.S. official told ABC News.

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"There's a lot that we're still trying to learn, but what we do know is that one of our service members who was on a tour willfully, and without authorization, crossed the military demarcation line. We believe that he is in DPRK custody," Secretary of Defense Lloyd Austin told reporters. "And so we're closely monitoring and investigating the situation and working to notify the soldier's next of kin and engaging to address this incident."

"The U.S. is working with the North Korean military to resolve this issue," Col. Isaac Taylor, a spokesman for U.S. Forces Korea, said in a statement.

King was reportedly released from a South Korean detention facility, where he spent about two months following an altercation with locals, and then spent about a week on a U.S. base under observation there, per ABC News. He finished the outprocessing procedures on Monday and was supposed to fly back to the United States, but he didn’t get on the flight.

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Since King had served his time and there was no indication he wouldn’t get on the flight, there was no escort to see him not get on the plane.

The Joint Security Area is a part of the wider Demilitarized Zone between South and North Korea, and tours of the area are open to the public and organized by the United Nations Command, which said in a statement: “A U.S. National on a JSA orientation tour crossed, without authorization, the Military Demarcation Line into the Democratic People’s Republic of Korea (DPRK)."