For most eavesdropping on embassies, federal law requires the F.B.I. to obtain an order from the Foreign Intelligence Surveillance Court, which meets in secret at a federal courthouse in Washington. If an American visiting or calling an embassy turns up on a recording, the F.B.I. is required by law to remove the American’s name from intelligence reports, substituting the words “U.S. person.” But raw transcripts would not necessarily have undergone such editing, called “minimization.”

Mr. Silverstein’s account could not be fully corroborated, but it fits the publicly known facts about the case. Spokesmen for the F.B.I., the Justice Department and the Israeli Embassy declined to comment on either eavesdropping on the embassy or Mr. Leibowitz’s crime. He admitted disclosing “classified information concerning the communication intelligence activities of the United States,” standard language for the interception of phone calls, e-mails and other messages by the F.B.I. and the National Security Agency, which generally focuses on international communications.

Mr. Leibowitz, now in a Federal Bureau of Prisons halfway house in Maryland, is prohibited by his plea agreement from discussing anything he learned at the F.B.I. Two lawyers who represented Mr. Leibowitz, Cary M. Feldman and Robert C. Bonsib, also would not comment.

Mr. Silverstein, 59, writes a blog called Tikun Olam, named after a Hebrew phrase that he said means “repairing the world.” The blog gives a liberal perspective on Israel and Israeli-American relations. He said he had decided to speak out to make clear that Mr. Leibowitz, though charged under the Espionage Act, was acting out of noble motives. The Espionage Act has been used by the Justice Department in nearly all prosecutions of government employees for disclosing classified information to the news media, including the record-setting five such cases under President Obama.

Image Shamai K. Leibowitz in 2002. He leaked secret transcripts. Credit... Peter Dejong/Associated Press

Mr. Silverstein said he got to know Mr. Leibowitz, a lawyer with a history of political activism, after noticing that he, too, had a liberal-minded blog, called Pursuing Justice. The men shared a concern about repercussions from a possible Israeli airstrike on nuclear facilities in Iran. From his F.B.I. work from January to August of 2009, Mr. Leibowitz also believed that Israeli diplomats’ efforts to influence Congress and shape American public opinion were excessive and improper, Mr. Silverstein said.

“I see him as an American patriot and a whistle-blower, and I’d like his actions to be seen in that context,” Mr. Silverstein said. “What really concerned Shamai at the time was the possibility of an Israeli strike on Iran, which he thought would be damaging to both Israel and the United States.”