The recent briefings of Deputy Defense Secretary Paul Wolfowitz and Douglas Feith, the undersecretary for policy, could indicate that the FBI probe is nearing a conclusion. No arrests have been made or charges announced.
Lesley Stahl of 60 Minutes broke the story last week.
The central figure in the investigation is Larry Franklin, a Middle East analyst who works in Feith's office. He has not responded to telephone messages seeking comment.
Investigators are trying to determine whether Franklin passed highly classified material about Bush administration policy on Iran to the American Israel Public Affairs Committee, the major Israeli lobbying group in Washington, and whether AIPAC in turn passed it to Israel.
CBS News Correspondent Jim Stewart reports that the question most puzzling the FBI at this point is whether Israeli agents were pushing Franklin for information, or whether the analyst was offering it on his own. The distinction could mean the difference between charges of unlawful disclose of classified material or espionage.
U.S. officials confirm the investigation has been going on for over a year, and that Franklin had recently agreed to cooperate with the FBI. The story was first reported last Friday on the CBS Evening News shortly after agents began executing search warrants. U.S. law enforcement officials did not object to the timing of the CBS report.
AIPAC has acknowledged that the FBI has interviewed some of its employees but strongly denies any wrongdoing.
"We've cooperated in this investigation. We think there is nothing to it," said AIPAC attorney Nathan Lewin.
Israeli Foreign Minister Silvan Shalom said Monday that meetings between Israeli embassy employees in Washington and U.S. government officials are commonplace, and that the two governments routinely share secrets.
"Israel and the United States have intimate ties ... and the information being exchanged is much more classified than any conversation that may have taken place," Shalom told reporters in Israel.
The U.S. officials familiar with Feith's briefing, which occurred Sunday at his office in the Pentagon, said he was told the investigation's focus inside the Defense Department was limited to Franklin.
Other senior Defense Department and State Department officials have also been interviewed or briefed by the FBI, but the officials would provide no other names.
So far, little light has been shed on what might have motivated Franklin to allegedly pass on the secret Iran material.
Israel has said Iran's ambitions to become a nuclear power pose the single greatest threat to the Jewish state. The United States has strongly backed an Israeli effort to block nuclear development in Iran, with President Bush including Iran in his "axis of evil" along with Iraq and North Korea.
AIPAC, which is instrumental in procuring some $3 billion in annual U.S. aid to Israel, lists stopping the Iranian threat as one of its top priorities. AIPAC boasts more than 65,000 members in all 50 states.
Also Monday, Israeli officials confirmed that a senior Israeli diplomat in Washington has met with Franklin. Those officials, also speaking on condition of anonymity, identified the diplomat as Naor Gilon, head of the Israeli Embassy's political department.
Gilon told the Israeli newspaper Maariv that he did nothing wrong but was concerned that he may no longer be able to work in Washington because of the investigation.