However, what remains unclear -- in light of a state-televised confession Monday of the alleged leader, Hamid "Dani" Tefileen -- is whether Tefileen will take the fall for the entire group, or if his confession will provide ammunition for punishing some or all of the accused.
On Wednesday, a second and third prisoner "confessed" to spying for Israel and its foreign intelligence agency, Mossad, "out of love" for the Jewish state. Western observers, however, charge that the confessions were either extracted under duress, with promises of leniency -- or both.
To the despair of family, friends and American Jewish advocates of the "Iran 13," no representatives of the media, human rights organizations or Jewish groups were allowed to monitor Monday's first-day proceedings.
The reason, said officials of Iran's Revolutionary Court, was fear for "national security."
In this and all Iranian court cases, the judge also acts as investigator, prosecutor and jury, which observers say is clearly a recipe for judicial abuse.
Monday's hearing -- which had been postponed from April 13 until after Passover -- did little to assuage that concern.
In his confession, Tefileen, a devoutly religious man from the southern city of Shiraz, admitted to visiting Israel in 1994.
Iran has denounced Israel as its archenemy. After their arrest more than one year ago, the "Iran 13" -- including religious and community leaders and one teenager -- were also accused of spying for the United States. Only Israel was mentioned Monday.
Interviewed by state television after Monday's hearing, Tefileen reportedly also admitted to being trained and paid by the Mossad, the Israeli intelligence agency. After the hearing, the authorities assailed him for carrying out this work for "the love of Israel" -- and for the money.
Israel, for its part, steadfastly denies the link.
"We don't have anything new to say from what we have said in the past," Israeli Foreign Ministry spokesman Aviv Sharon told reporters Monday in Israel.
"We don't have any connection with any of those who stand trial now in Iran."
But it is the Israeli stamp in Tefileen's passport that seems to be, from Iran's perspective, the smoking gun for the entire case, said Malcolm Hoenlein, executive vice chairman of the Conference of Presidents of Major American Jewish Organizations.
"Without him, there's no case," said Hoenlein, who has been closely monitoring developments taking place in Iran. "Getting him to confess was crucial."
Tefileen has not yet been sentenced, though state television said he had asked for clemency.
Hearings for the remaining 12 are likely to continue during the next two weeks.
Their fate is virtually impossible to predict, said Hoenlein, as Iranian judiciary officials have not yet produced any evidence. At the same time, they routinely flout Iranian law and demonstrate a blatant disregard for international public opinion, he said.
Iran has been universally condemned for what is widely viewed as a show trial against the 13 Jews.
Why, then, did Tefileen confess?
Observers believe that Tefileen, an impoverished trader, may have been facing other trumped-up charges, like drug possession or smuggling.
He may have been offered leniency if he confessed to the spying charge. Or perhaps the freedom of his brother, Omid, was dangled before him.
Omid is one of three accused Jews who was released on bail in February and is likely to be pardoned.
Either way, said Hoenlein, Tefileen and the rest are innocent.
"If this doesn't give all the appearances of a setup, I don't know what is," said Hoenlein. "Listen, you sit in jail for a year, in conditions no one knows about. How can we judge someone in that situation?"
Despite the Iranian authorities' apparent self-satisfaction at having their man, Tefileen's court-appointed lawyer, Shirzad Rahmani, told the Associated Press that the confession is not enough to convict.
Under Iranian law, Rahmani noted, the state has to prove its case with evidence.
"There may have been confessions, there may have been an intention to spy, there may have been several trips to Israel, and there may have been payments," he told the AP. "But if information damaging to Iran and beneficial to Israel was not actually exchanged, there can be no charge of espionage."