In a lengthy letter to supporters and allies, the American Israel Public Affairs Committee charged yesterday that the "very essence" of relations between the United States and Israel is under assault as a result of reports that the FBI is investigating whether AIPAC officials passed classified information to Israel.

The e-mail, AIPAC's first detailed comment about the FBI's counterintelligence investigation, strongly denies involvement in criminal activity or receipt of secret intelligence information by the organization or its employees, calling the allegations "false and baseless."

It also appeals to supporters for a "special contribution" to combat the publicity, including a recent full-page ad in the Washington Times that alleges that the United States has turned a blind eye to Israel's violations of international law.

The AIPAC letter comes amid charges from some American Jews that enemies of Israel and of AIPAC are capitalizing on the controversy created by reports of the probe. Some have said their goal is to undermine relations with Israel. Others are concerned about an anti-Semitic undercurrent because many of the U.S. officials that the FBI has mentioned in interviews are Jews in the Pentagon and Vice President Cheney's office.

Others say the probe is rooted in political and ideological conflict.

"There's a political component to this. What we've had is the criminalization of the foreign policy debate. There are accusations being floated around more often based on policy disagreement rather than fact," said Michael Rubin, resident scholar at the American Enterprise Institute and a former Iran and Iraq specialist in the Pentagon's policy office. The FBI has declined to comment on the counterintelligence investigation, which differs from a criminal probe in that it may not result in criminal charges. Its scope and specifics are largely unknown, though sources familiar with the case have said it began more than two years ago.

A criminal investigation is also underway into whether Lawrence A. Franklin, a Pentagon policy analyst, provided a draft presidential directive on Iran to AIPAC, and whether AIPAC passed the information to Israel, according to sources who have been interviewed in the case.

Over the weekend, Israeli Ambassador Daniel Ayalon said on "John McLaughlin's One on One" that the investigation was the result of either malicious intent or "some incompetence of not understanding reality," a reference to long-standing U.S.-Israeli cooperation.

Administration officials, people interviewed in the probe and Jewish groups are anxious for the probe to conclude. "Everyone is waiting to see if there is a reality to this: Are people going to start to get arrested or indicted? We don't know what is going on behind the curtain," said one person who was interviewed by authorities. "In the not-too-distant future, there's going to have to be something more definitive."

In the letter, AIPAC President Bernice Manocherian and Executive Director Howard Kohr said the group will not "abide any suggestion that American citizens should be perceived as being involved in illegal activities simply for seeking to participate in the decisions of their elected leaders or officials who work for them."

It also charged that the ad in the Washington Times, sponsored by the Council for the National Interest, was an attempt to convince policymakers that AIPAC "is doing something wrong." The ad criticized Israel for conducting espionage and covert operations against the United States, erecting an "apartheid wall" to separate Israel from Palestinians and building illegal settlements.

An AIPAC official said yesterday that public response to the e-mail, which includes statements of support from congressional leaders, had been "exceptional." Heavy traffic briefly overwhelmed AIPAC's Web site.

The letter also seeks to shift the public focus. "We would prefer to be talking about Israel's disengagement plan, that Iran may be mere months away from nuclear weapons, the injustices toward Israel in the United Nations, but our institution and the very essence of the United States-Israel relationship is under assault," said Josh Block, AIPAC spokesman.