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FBI Director Robert S. Mueller III told Congress on Thursday his agency needs additional agents, money and time as it works to meet its "paramount mission of prevention" in an age of terrorism.

"The FBI must become better at shaping its workforce, collaborating with its partners, applying technology to support investigations, operations and analyses, protecting our information and developing core competencies," Mueller said in remarks prepared for delivery to the Senate Judiciary Committee.

Mueller was speaking as President Bush took steps to overhaul the nation's system for terrorism security, nine months after the attacks that killed thousands in New York, Washington and Pennsylvania. Bush was expected to use a prime time address to propose the creation of an agency to act as a clearinghouse for terrorism intelligence, according to several White House officials who called the effort one of the biggest restructuring efforts since World War II.

Sen. Patrick Leahy, D-Vt., the committee chairman, complimented Mueller for his candor in responding to the terrorist attacks, particularly for conceding that it is impossible to say if the Sept. 11 attacks might have been prevented if "all the dots had been connected and all the leads had been followed."

At the same time, Leahy expressed unhappiness that Congress had not been informed about the existence of a memo alerting FBI officials that several Arabs were suspiciously training at a U.S. aviation school in Arizona.

Apart from Mueller and Justice Department Inspector General Glenn Fine, the panel was hearing from Coleen Rowley, an FBI lawyer in Minneapolis who sent the FBI director a scathing letter on May 21 saying he and other senior FBI officials had skewed facts in their post-Sept. 11 accounts of what they had known before the attacks and were trying to "circle the wagons."

Rowley also claimed FBI headquarters shelved her requests in the weeks before the attacks to aggressively investigate Zacarias Moussaoui, who was being held in Minnesota and now is charged as an accomplice in the hijackings that killed more than 3,000 people.

"The people (in the FBI) that knew what was going on or had suspicions what was going on did not brief him properly and their heads ought to roll," Sen. Charles Grassley, R-Iowa, said Thursday on CBS' "The Early Show."

Rowley was to testify before the Senate committee late Thursday.

The House and Senate intelligence committees, meanwhile, met Wednesday for a second day of closed-door investigations into Sept. 11, without calling any witnesses, while several staff members from their joint inquiry went to FBI headquarters to question Rowley.

The Bush White House favors the inquiry by the joint intelligence committees, but Leahy said he will press forward because of his committee's constitutional obligation to ensure the FBI is adequately protecting Americans.

"We have the ultimate responsibility to report on whether there were mistakes made," Leahy said.

Congressional aides, speaking on condition of anonymity, said Judiciary Committee Democrats planned to press Justice Department Inspector General Glenn Fine on when he knew about the so-called Phoenix memo.

The aides say Fine knew about that memo on Sept. 29, days after the attacks. Congress found out about the memo, called an electronic communication, or EC, last month.

The Justice Department's inspector general's office "conducted a preliminary inquiry in the fall of 2001 into the handling of the Phoenix EC at FBI headquarters," Fine confirmed in prepared testimony provided to The Associated Press.

Paul Martin, spokesman for the inspector general, said seven interviews on the Phoenix memo were conducted with FBI headquarters officials in November and December. In his prepared testimony, Fine said the IG then referred the matter to the congressional intelligence committees.

The Washington headquarters agent to whom the memo was addressed, David Frasca, says he didn't see the document until after Sept. 11, when shown it by Justice Department investigators.

The Senate committee also was focusing on the reorganization plan announced by Mueller last week that would turn the agency more toward terrorism prevention and on new rules issued by Attorney General John Ashcroft giving the FBI more leeway to monitor public places.

The bureau plans to move a number of key decision-making powers away from headquarters to the field offices and wants to hire some 900 new agents by September, mostly specialists in computers, foreign languages and sciences. Hundreds of agents will be reassigned to counterterrorism.

In an interview in Thursday's editions of The Washington Post, Mueller said the FBI has placed a substantial number of people who may have ties to al-Qaida under 24-hour surveillance throughout the country, a mission that has "really pushed" the bureau.

"Our biggest problem is we have people we think are terrorists. They are supporters of al-Qaida," he said. "They may have been sworn jihad (holy war), they may be here in the United States legitimately and they have committed no crime."

Mueller also tried to alleviate Arab-Americans' concerns that the FBI would spy unnecessarily on mosques and heighten fear of Muslims.

"We don't have a plan to go into mosques," Mueller told the Post. "We take each investigation on its own and look at it and then what's appropriate for the investigation."

Thursday's hearing follows recent news reports that the FBI and CIA failed to respond adequately to warning signs of possible terrorist activity before Sept. 11, including the Moussaoui arrest and information developed by the CIA in early 2000 about two of the hijackers.

FBI headquarters, before Sept. 11, rejected field agents' request to seek a search warrant for Moussaoui's computer. A search done after the Sept. 11 attacks found information on jetliners and crop-dusters. The government grounded crop-dusting planes temporarily because of what it found.