WASHINGTON — When Shamai K. Leibowitz, an F.B.I. translator, was sentenced to 20 months in prison last year for leaking classified information to a blogger, prosecutors revealed little about the case. They identified the blogger in court papers only as “Recipient A.” After Mr. Leibowitz pleaded guilty, even the judge said he did not know exactly what Mr. Leibowitz had disclosed.

“All I know is that it’s a serious case,” Judge Alexander Williams Jr., of United States District Court in Maryland, said at the sentencing in May 2010. “I don’t know what was divulged other than some documents, and how it compromised things, I have no idea.”

Now the reason for the extraordinary secrecy surrounding the Obama administration’s first prosecution for leaking information to the news media seems clear: Mr. Leibowitz, a contract Hebrew translator, passed on secret transcripts of conversations caught on F.B.I. wiretaps of the Israeli Embassy in Washington. Those overheard by the eavesdroppers included American supporters of Israel and at least one member of Congress, according to the blogger, Richard Silverstein.

In his first interview about the case, Mr. Silverstein offered a rare glimpse of American spying on a close ally.

He said he had burned the secret documents in his Seattle backyard after Mr. Leibowitz came under investigation in mid-2009, but he recalled that there were about 200 pages of verbatim records of telephone calls and what seemed to be embassy conversations. He said that in one transcript, Israeli officials discussed their worry that their exchanges might be monitored.

Mr. Leibowitz, who declined to comment for this article, released the documents because of concerns about Israel’s aggressive efforts to influence Congress and public opinion, and fears that Israel might strike nuclear facilities in Iran, a move he saw as potentially disastrous, according to Mr. Silverstein.

While the American government routinely eavesdrops on some embassies inside the United States, intelligence collection against allies is always politically delicate, especially one as close as Israel.

The Federal Bureau of Investigation listens in on foreign embassies and officials in the United States chiefly to track foreign spies, though any intelligence it obtains on other matters is passed on to the C.I.A. and other agencies. The intercepts are carried out by the F.B.I.’s Operational Technology Division, based in Quantico, Va., according to Matthew M. Aid, an intelligence writer who describes the bureau’s monitoring in a book, “Intel Wars,” scheduled for publication in January. Translators like Mr. Leibowitz work at an F.B.I. office in Calverton, Md.

Former counterintelligence officials describe Israeli intelligence operations in the United States as quite extensive, ranking just below those of China and Russia, and F.B.I. counterintelligence agents have long kept an eye on Israeli spying.

For most eavesdropping on embassies in Washington, federal law requires the F.B.I. to obtain an order from the Foreign Intelligence Surveillance Court, which meets in secret at the Justice Department. 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.

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.”

Mr. Silverstein took the blog posts he had written based on Mr. Leibowitz’s material off his site after the criminal investigation two years ago. But he was able to retrieve three posts from April 2009 from his computer and provided them to The New York Times.

The blog posts make no reference to eavesdropping, but describe information from “a confidential source,” wording Mr. Silverstein said was his attempt to disguise the material’s origin.

One post reports that the Israeli Embassy provided “regular written briefings” on Israel’s war with Hamas in Gaza to President Obama in the weeks between his election and inauguration. Another describes calls involving Israeli officials in Jerusalem, Chicago and Washington to discuss the views of members of Congress on Israel. A third describes a call between an unnamed Jewish activist in Minnesota and the Israeli Embassy about an embassy official’s meeting with Representative Keith Ellison, Democrat of Minnesota, who was planning an official trip to Gaza.

Mr. Silverstein said he remembered that embassy officials talked about drafting opinion articles to be published under the names of American supporters. He said the transcripts also included a three-way conversation between a congressman from Texas, an American supporter of the congressman and an embassy official; Mr. Silverstein said he could not recall any of the names.

At his sentencing, Mr. Leibowitz described what he had done as “a one-time mistake that happened to me when I worked at the F.B.I. and saw things which I considered were violation of the law, and I should not have told a reporter about it.”

That was a reference to Israeli diplomats’ attempts to influence Congress, Mr. Silverstein said, though nothing Mr. Leibowitz described to him appeared to be beyond the bounds of ordinary lobbying.

Mr. Leibowitz, 40, the father of 6-year-old twins at the time of sentencing, seems an unlikely choice for an F.B.I. translation job. He was born in Israel to a family prominent in academic circles. He practiced law in Israel for several years, representing several controversial clients, including Marwan Barghouti, a Palestinian leader convicted of directing terrorist attacks on Israelis, who Mr. Leibowitz once said reminded him of Moses.

In 2004, Mr. Leibowitz moved to Silver Spring, Md., outside Washington, where he was a leader in his synagogue. Mr. Silverstein said Mr. Leibowitz holds dual American and Israeli citizenship.

In court, Mr. Leibowitz expressed anguish about the impact of the case on his marriage and family, which he said was “destitute.” He expressed particular sorrow about leaving his children. “At the formative time of their life, when they’re 6 years old and they’re just finishing first grade, I’ll be absent from their life, and that is the most terrible thing about this case,” he said.

While treated as highly classified by the F.B.I., the fact that the United States spies on Israel is taken for granted by experts on intelligence. “We started spying on Israel even before the state of Israel was formally founded in 1948, and Israel has always spied on us,” said Mr. Aid, the author. “Israeli intercepts have always been one of the most sensitive categories,” designated with the code word Gamma to indicate their protected status, he said.

Douglas M. Bloomfield, an American columnist for several Jewish publications, said that when he worked in the 1980s for the American Israel Public Affairs Committee, a lobbying group, he assumed that communications with the embassy were not private.

“I am not surprised at all to learn that the F.B.I. was listening to the Israelis,” he said. “But I don’t think it’s a wise use of resources because I don’t see Israel as a threat to American security.”