FBI Director Robert S. Mueller III thanked Muslim Americans yesterday for providing "substantive assistance" to the government investigation of the Sept. 11 attacks.

In a controversial appearance before the American Muslim Council, Mueller noted that some people associated with the group have made statements in the past that were supportive of terrorism.

But he said that the organization has moved into the mainstream of American life and that "we must all be in this war . . . together."

Two rabbis held signs protesting Mueller's appearance and were ushered out of the meeting of several hundred people in Alexandria. There were calls beforehand for Mueller to cancel the speech. The American Muslim Council's former executive director has been quoted as expressing support for the terrorist group Hamas.

Mueller defended his appearance, saying, "It is critically important for us to develop a strong relationship."

The United States has frozen the assets of some of the charities to which the council has urged its members to contribute, citing alleged links to terrorists.

Hahya Mossa Basha, the council's board chairman, said the council does not support terrorists.

One of the protesters, Rabbi Avi Weiss, called Mueller's appearance "a disgrace."

"The mission of the FBI is to protect Americans; it is not to have its director speak to an organization that has supported other organizations" linked to terrorism, Weiss said. He is president of the Coalition for Jewish Concerns and senior rabbi at the Hebrew Institute of Riverdale, N.Y.

Mueller said many Muslims in the United States have volunteered to serve as Arabic translators. That allowed the FBI to double the number of its Arabic experts and substantially reduce a backlog of items needing translating.

Some Arabic translators have gone to the U.S. naval base at Guantanamo Bay, Cuba, to help interview al Qaeda or Taliban suspects detained there, Mueller said.

Mueller sought to assure the group that his agency will not abuse broad new powers, granted to help it fight terrorism, by trampling on civil liberties or unfairly singling out Arabs or Muslims.

"We are out to address terrorists," Mueller said. "This is in no way a war against Islam."

The group's treasurer, Mohammed Ali Khan, said that in a short, closed-door session with Mueller, the group expressed its concern over the unknown number of detainees jailed in the attack investigation.

Khan said that the number of pleas for help to the organization from families of detainees has gone down somewhat since last fall but that the group is still receiving some every week.

Mueller also condemned attacks against members of the Muslim community in the wake of Sept. 11, saying "like terrorism, they are acts against humanity."

FBI Director Robert S. Mueller III addresses the American Muslim Council.