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Since his arrest in late September, Mr. Arbabsiar has “confessed to his participation in the alleged plot [and] provided other valuable information about elements of the Iranian government’s role in it,” Mr. Holder said.

According to court documents, Mr. Arbabsiar has identified several of the Iranian conspirators, discussed their motivation and their roles in the plot, and picked one of them out of a photo lineup.

“The key question here will be the exact role” of the Qods Force officials, especially Mr. Shalai, said Roger Cressey, who served on the White House national security staffs of Presidents Bill Clinton and George W. Bush. “Was he acting on his own or was he acting at the direction of his superiors in the Iranian government?”

“Corroborating evidence” for Mr. Arbabsiar’s confession will be “critical in building a public case in the international court of public opinion,” Mr. Cressey said.

“This revelation that Iran was actively plotting an attack on U.S. soil must serve as a wake-up call,” said Rep. Iliana Ros-Lehtinen, Florida Republican and chairman of the House Foreign Affairs Committee.

“Today’s news illustrates Iran’s active campaign to partner with extremist groups, drug traffickers and other outlaws based in the Western Hemisphere,” she said. “Responsible nations must unite against this threat and immediately bring to bear crippling pressure on the Iranian regime and its enablers.”

But Mr. Cressey said he remains skeptical about the possibility of international action against Iran in the plot.

“It is one thing to put together an international coalition in the aftermath of a terror attack, it’s quite different when the attack has been foiled, especially when, as in this case, it was a sting operation,” he said.

Secretary of State Hillary Rodham Clinton told reporters that U.S. officials “will be consulting with our friends and partners around the world about how we can send a very strong message that this kind of action, which violates international norms, must be ended.”

She promised measures to “further isolate [Iran] from the international community.”