The probe, which has been going on for more than two years, also has focused on other civilians in the Secretary of Defense's office, said the sources who have first-hand knowledge of the subject.
In addition, one said, FBI investigators in recent weeks have conducted interviews to determine whether Pentagon officials gave highly classified U.S. intelligence to a leading Iraqi exile group, the Iraqi National Congress, which may in turn have passed it on to Iran. INC leader Ahmed Chalabi has denied his group was involved in any wrongdoing.
The linkage, if any, between the two leak investigations, remains unclear.
But they both center on the office of Undersecretary of Defense Douglas Feith, the Pentagon's No. 3 official.
Feith's office, which oversees policy matters, has been the source of numerous controversies over the last three years. His office had close ties to Chalabi and was responsible for post-war Iraq planning that the administration has now acknowledged was inadequate. Before the war, Feith and his aides pushed the now-discredited theory that former Iraqi leader Saddam Hussein was in league with al-Qaida.
No one is known to have been charged with any wrongdoing in the current investigation.
Officials cautioned that it could result in charges of mishandling classified information, rather than the more serious charge of espionage.
The Israeli government on Saturday strenuously denied it had spied on the United States, its main benefactor on the global scene.
The American Israel Public Affairs Committee, the powerful pro-Israel lobby that top officials said is suspected of serving as a conduit to Israel for the midlevel analyst, also has denied any wrongdoing.
That analyst, Larry Franklin, works for Feith's deputy, William Luti, and served as an important - albeit low-profile - advisor on Iran issues to Feith and Deputy Defense Secretary Paul Wolfowitz.
Investigators are said to be looking at whether Franklin acted with authorization from his superiors, one official said.
Two sources disclosed Saturday that the information believed to have been passed to Israel was the draft of a top-secret presidential order on Iran policy, known as a National Security Presidential Directive. Because of disagreements over Iran policy among President Bush's advisors, the document is not believed to have ever been completed.
Having a draft of the document - which some Pentagon officials may have believed was insufficiently tough toward Iran - would have allowed Israel to influence U.S. policy while it was still being made. Iran is among Israel's main security concerns.
Two or three staff members of AIPAC have been interviewed in connection with the case.
In a prepared statement, AIPAC said any allegation of criminal conduct was ''false and baseless.'' It is ''cooperating fully,'' with investigators, AIPAC's statement said.
Israeli officials insisted they stopped spying on the United States after the exposure of Jonathan Pollard, who was arrested in 1985 and sentenced to life in prison for spying for Israel.