PYONGYANG, North Korea (AP) — North Korea’s Supreme Court on Sunday convicted a 24-year-old American man of entering the country illegally to commit espionage and sentenced him to six years of hard labor.

At a trial that lasted about 90 minutes, the court said Matthew Miller, of Bakersfield, California, tore up his tourist visa at Pyongyang’s airport upon arrival on April 10 and admitted to having the “wild ambition” of experiencing prison life so that he could secretly investigate North Korea’s human rights situation.

Miller, who looked thin and pale at the trial and was dressed completely in black, is one of three Americans being held in North Korea.

Showing no emotion throughout the proceedings, Miller waived the right to a lawyer and was handcuffed before being led from the courtroom after his sentencing. The court, comprising a chief judge flanked by two “people’s assessors,” ruled it would not hear any appeals to its decision.

Earlier, it had been believed that Miller had sought asylum when he entered North Korea. During the trial, however, the prosecution argued that was a ruse and that Miller also falsely claimed to have secret information about the U.S. military in South Korea on his iPad and iPod.

Miller was charged under Article 64 of the North Korean criminal code, which is for espionage and can carry a sentence of five to 10 years, though harsher punishments can be given for more serious cases.

The Associated Press was allowed to attend the trial.

A trial is expected soon for one of the other Americans being held, Jeffrey Fowle, who entered the North as a tourist and was arrested in May for leaving a Bible at a sailor’s club in the city of Chongjin. The third American, Korean-American missionary Kenneth Bae, is serving out a 15-year sentence for alleged “hostile acts.”

All three have appealed to the U.S. government to send a senior statesman to Pyongyang to intervene on their behalf.

During a brief interview with The Associated Press in Pyongyang last week, Miller said he had written a letter to President Barack Obama but had not received a reply.

Following Sunday’s court verdict, the U.S. State Department urged North Korea to release Miller, as well as Bae and Fowle.

“Now that Mr. Miller has gone through a legal process, we urge the DPRK to grant him amnesty and immediate release,” State Department spokeswoman Jen Psaki said in a statement, using North Korea’s official name, the Democratic People’s Republic of Korea.

Fowle, a 56-year-old equipment operator for the city of Moraine, Ohio, said his wife, a hairstylist from Russia, made a written appeal on his behalf to Russian President Vladimir Putin. He said the Russian government responded that it was watching the situation.

The U.S. has repeatedly offered to send its envoy for North Korean human rights issues, Robert King, to Pyongyang to seek the freedom of the detainees, but without success.

Former President Bill Clinton came in 2009 to free a couple of jailed journalists. Jimmy Carter made the trip in 2010 to secure the release of Aijalon Gomes, who had been sentenced to eight years of hard labor for illegally crossing into the country to do missionary work.

In 2011, the State Department’s envoy for North Korean human rights managed to successfully intervene in the case of Korean-American businessman Eddie Yong Su Jun.

The United States has no diplomatic relations with North Korea and strongly warns American citizens against traveling to the country.

Uri Tours, a New Jersey-based travel agency specializing in North Korea tourism that handled the arrangements for Miller, said in an email Sunday that it was working to have Miller returned to his parents in the United States.

“Although we ask a series of tailored questions on our application form designed to get to know a traveler and his/her interests, it’s not always possible for us to foresee how a tourist may behave during a DPRK tour,” the travel agency said in a statement on Friday. “Unfortunately, there was nothing specific in Mr. Miller’s tour application that would have helped us anticipate this unfortunate outcome.”

The agency said that as a result of the incident, it now routinely requests a secondary contact and reserves the right to contact those references to confirm facts about a potential tourist. It has also added advice warning tourists not to rip up any officially issued documents and “to refrain from any type of proselytizing.”

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Associated Press National Security Writer Lara Jakes contributed to this report from Paris.