The trail of three American hikers accused of being spies began in Iran Sunday. The young friends pleaded not guilty to charges of espionage and trespassing, according to Iranian state television.

Two of the three Americans, Shane Bauer and Josh Fattal, appeared in court. They were arrested by border guards July 31, 2009, when found near the Iranian border with Iraq along with Sarah Shourd, who returned home in September after being released on $500,000 bail.

The three have claimed they were hiking in the mountains and thought they were in Iraq. 

If convicted, espionage can carry the death penalty.

State television showed the two men looking "gaunt and worried," AP reports.

Iran barred international observers and journalists from the opening of the trial. It also banned Switzerland's ambassador, Livia Leu Agosti, who represents U.S. interests in Iran.

The Independent reports that the defendants' lawyer was not allowed to meet with the men until a few hours before the trial began. However, The New York Times reports that the lawyer did not get any chance to meet with his clients.

The three Americans are graduates of the University of California at Berkeley. Shourd and Bauer had been living together in Damascus, Syria, where Bauer was working as a freelance journalist and Shourd as an English teacher, according to the Associated Press. Fattal, an environmental activist, went to visit them in July 2009.

The episode has put a strain on an already tense relationship between the United States and Iran. The two countries have not had diplomatic relations since the 1979 Islamic Revolution and storming of the U.S. embassy.

There has been a worldwide campaign calling for the release of the hikers along with emotional appeals from the families.

Shourd, who refused to return to Iran to stand trial, posted a message on a campaign website calling for their release.

"Shane and Josh don't deserve to be in prison one minute longer that I was, and never deserved to be there in the first place," she wrote. "We were living, working and traveling in the region in order to increase our knowledge of its diverse cultures, lend a hand through our humanitarian work and promote more understanding in our communities back home."

The trial was originally planned to begin in November but was delayed after Shourd refused to return. She is now being tried in absentia.

No date has been set for the next hearing.

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