Story highlights Washington calls for the immediate release of Miller and Kenneth Bae

A court says Matthew Todd Miller committed "acts hostile" to North Korea

He previously told CNN, "I deliberately committed my crime"

That interview was monitored by the North Korean government

For months he was held captive by the North Korean regime, not even sure what he would be charged with.

Then in one fell swoop, American citizen Matthew Todd Miller was convicted of committing "acts hostile" to North Korea and sentenced to six years of hard labor, North Korean state-run media reported Sunday.

While Miller's fate is now clear, the circumstances surrounding his alleged crime remain murky.

According to the state-run Korean Central News Agency, the 24-year-old arrived in North Korea as a tourist on April 10 and ripped up his tourist visa, shouted his desire to seek asylum and said "he came to the DPRK after choosing it as a shelter."

DPRK refers to North Korea's official name, the Democratic People's Republic of Korea. Its government is a communist dictatorship renowned for human rights abuses.

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Washington demands release

Early Monday, the U.S. State Department demanded that North Korea release Miller and fellow American prisoner Kenneth Bae.

"The charges for which he (Miller) and the other detained U.S. citizens were arrested and imprisoned would not give rise to arrest or imprisonment in the United States or in many other countries around the world," said spokesman Darby Holladay.

He also asked for amnesty for the men and for American Jeffrey Fowle.

Fowle, 56, was arrested in June while traveling as a tourist. Holladay said that U.S. citizens are discouraged from visiting the country.

Miller talks to CNN

Earlier this month, Miller told CNN's Will Ripley that he "prepared to violate the law of DPRK before coming here. And I deliberately committed my crime."

But Miller didn't elaborate on what his "crime" was. He said he wouldn't learn of his charges until he went to trial.

It's unclear whether his statements were made freely or under coercion.

The North Korean government surprised CNN by presenting Miller and two other detained Americans -- Kenneth Bae and Jeffrey Edward Fowle -- for interviews. Each five-minute interview was monitored by the government.

"My situation is very urgent," Miller told CNN. "... Very soon, I'm going to trial and I will directly be sent to prison. I think this interview is my final chance to push the American government into helping me."

A quiet California kid

Miller's family lives in Bakersfield, California. Close friends and neighbors told CNN they were instructed by the family not to speak to reporters.

Miller is a 2008 graduate of Bakersfield High School, according to CNN affiliate KBAK

A few classmates told CNN that Miller seemed like an average kid. Two said they barely remembered him because he was so quiet.

In a July interview, a neighbor told The Associated Press that Miller went to South Korea about four years ago to visit his brother and that he found a job teaching English.

He traveled to North Korea this year after arranging a private tour through the U.S.-based company Uri Tours, which takes tourists into North Korea.

Uri Tours has said it doesn't have "any understanding of why" Miller ripped up his visa.

The company offers tours despite U.S. State Department warnings that U.S. citizens have been subject to arbitrary arrest and detention in North Korea.

Miller's case presents many questions, said Robert Kelly, an American who teaches International studies at Pusan National University in South Korea.

"If he wanted asylum, why's he trying to get out?" Kelly asked. "Now, he changes his mind? This is why the (U.S.) State Department encourages citizens not to go to North Korea."