Tehran, Iran (CNN) -- The trial of three U.S. hikers started Sunday in Iran, according to the office of the lawyer representing them.

Iran accuses Americans Shane Bauer, 28, Josh Fattal, 28, and Sarah Shourd, 32, of spying and trespassing.

They were detained July 31, 2009, after they allegedly strayed across an unmarked border into Iran while hiking in Iraq's Kurdistan region.

Shourd was released on bail in September 2010 because of a medical condition and immediately left the country. She has not responded to a court summons to return to stand trial, lawyer Masoud Shafii said Saturday.

Iranian authorities said she will be tried in absentia if she doesn't appear in court.

The trial is closed to the press and the public, as is normally the case with revolutionary court proceedings. Iranian state media reported Sunday that not-guilty pleas had been entered for the three hikers.

The Swiss ambassador to Iran, who represents American interests in the country, told CNN that Sunday's trial will likely not continue the next day, but at a later date.

"It's going to be soon," Ambassador Livia Lea Agosti said, though she declined to divulge her source. "It's not going to be another three-month wait."

Agosti was not invited to attend the hikers' trial but showed up anyway, she said. She was not able to enter the courtroom but she put in a request to see Bauer and Fattal, according to official IRNA news agency. The pair were present in the courtroom for the proceedings, she told CNN.

Agosti also said that Shourd's decision not to return to Iran to stand trial was her own.

"This was Shourd's personal decision and I don't have any information as to why she didn't appear for the trial," the ambassador said, according to IRNA.

Shafii, the attorney, said he had been denied permission to see Bauer and Fattal the day before the trial began. He told CNN he has reviewed his clients' case file and doesn't see any evidence of a crime.

"In my opinion, they haven't done anything wrong," Shafii said. "The accusation of spying is baseless, and if they trespassed into Iran, it wasn't their fault."

Shafii said the border area where the hikers are accused of trespassing is unmarked and anyone could unwittingly cross over into Iran.

Human rights groups have condemned their arrests and their lengthy wait for a trial in Tehran's notorious Evin prison.

CNN's Reza Sayah contributed to this report