A U.S. soldier who darted across the border from South Korea was detained in North Korea, in a rare case of an American service member being held in one of the world's most isolated states. The incident has sparked a diplomatic emergency at a time of already strained ties between Pyongyang and Washington.

U.S. authorities identified the American as Army Pvt. Travis T. King and said he was believed to be in North Korean custody. They said he was punished for misconduct while serving in South Korea and was being sent back to the United States, when he skipped his flight.

Here's what we know so far:

Who is Army Private 2nd Class Travis King?

A cavalry scout, King joined the military in January 2021. U.S. officials have not publicly detailed the misconduct for which he was punished while serving in South Korea.

U.S. military personnel escorted him to the airport to be flown to the United States, but he did not board the scheduled flight, a U.S. official told The Washington Post, speaking on the condition of anonymity because of the matter's sensitivity. King, who has not served on combat deployments, has medals listed in his provided service record that were perfunctory awards given to soldiers in South Korea, The Post reported.

According to the Associated Press, the 23-year-old was recently released from a South Korean prison, where he had been held on assault charges. The AP reported that he was due to be sent this week to Fort Bliss, in Texas, where he could have faced further disciplinary actions and discharge from the service.

Why did he cross into North Korea?

King's motives were not immediately clear. He was visiting the Joint Security Area (JSA) separating North and South Korea while on an "orientation tour," Army Col. Isaac Taylor, a spokesman for U.S. forces in South Korea, said in a statement.

"We're very early in this event, and so there's a lot that we're still trying to learn," Defense Secretary Lloyd Austin said Tuesday. The service member "willfully and without authorization" crossed the military demarcation line during a tour, Austin told reporters at the Pentagon, adding: "We will remain focused on this."

The White House said U.S. agencies and the United Nations were "all working together to ascertain more information and resolve the situation."

What is the Joint Security Area?

Overseen by the United Nations, the frontier area that straddles North and South Korea is one of the most fortified in the world. The JSA is a section of the demilitarized zone, which has separated the North and South since the 1950-1953 Korean War ended in an armistice.

The JSA has been used for diplomatic discussions, and it's the only place where North Korean and South Korean guards stand face to face, near light-blue buildings on each side of the line.

The JSA in the border village of Panmunjom offers a rare window for visitors into what is technically North Korean and South Korean territory.

Koreans and tourists can visit fourteen "Education and Orientation Program sites" within the demilitarized zone to learn about the Korean War and armistice agreement, according to the United Nations Command.

What happened to Americans held in North Korea before?

King is the first American known to be detained in North Korea in nearly five years. North Korea has not commented on the incident so far.

Most releases of Americans detained after voluntarily entering the country have come from high-level talks between government officials.

The State Department has imposed a ban on U.S. nationals traveling to North Korea since 2017, after the death of Otto Warmbier. The American student, who traveled to Pyongyang and was detained on charges of stealing a propaganda poster, died soon after being released to U.S. custody and sent home in a coma.

Because there are no diplomatic or consular relations, the U.S. government cannot provide emergency services to its nationals in North Korea, the State Department notes in its travel advisory.

Among the instances of Americans held in North Korea in recent years was Bruce Byron Lowrance, who was detained in North Korea in October 2018 after crossing from China, North Korea's news agency claimed. He was deported to the United States in November 2018, after a historic meeting in Singapore that year between President Donald Trump and North Korean leader Kim Jong Un.

The Washington Post's Alex Horton, Michelle Ye Hee Lee, Niha Masih, Kelly Kasulis Cho and Leo Sands contributed to this report.