FBI Director Robert Mueller has urged the U.S. Congress not to weaken a key legal tool that law enforcement authorities have been using since September 11, 2001 in their efforts to prevent new terrorist attacks. VOA's Dan Robinson reports from Capitol Hill.

In his first appearance this year before a Democrat-controlled House Judiciary Committee, Director Mueller was pressed on a range of issues, including FBI reforms, the recent National Intelligence Estimate on terrorist threats, and congressional concerns about tactics used in the anti-terror fight.

Since the September 11, 2001 al-Qaida attacks in the United States, Mueller says the FBI has conducted thousands of investigations, involving persons he describes as being in some way associated with various Shia or Sunni terrorist groups, as well as funding and radicalization of potential terrorists.

Mueller was asked about the recent government intelligence report on al-Qaida capabilities. "For the next three years we face a threat of terrorist attack. In part it is attributable to the understanding [of] persons affiliated, associated with al-Qaida that it is important to try to find individuals who can circumvent our border security and come into the United States much the way the 19 hijackers did prior to September 11. They have since September 11 not for one instant given up the hope and the efforts to try to infiltrate persons into the United States to undertake attacks," he said.

One of the most controversial tools the FBI has used for decades is the National Security Letter, a form of subpoena used to demand that private records and communications be turned over to authorities from individuals.

In approving the Patriot Act law in 2001, Congress expanded the scope of National Security Letters, which have also been used by other government agencies, to cover residents of the U.S. or visitors.

But a recent internal FBI audit found that some FBI agents violated rules governing National Security Letters (N.S.L.), although the audit found no evidence of intentional violations by FBI agents.

Judiciary chairman John Conyers described the matter as an unfortunate chapter for the FBI.

"There are those in the Congress that begin to question the continued giving of the FBI this broad NSL authority in light of the findings, not only from the inspector general's report but from the FBI's own internal audit, that the bureau has systemic difficulties in limiting National Security Letters to appropriate uses," he said.

Referring to recent terrorist investigations in Britain, information from which also went to U.S. authorities tracking potential terrorists, Mueller says National Security Letters remain a crucial part of an across the board anti-terror effort.

He sought to assure lawmakers about FBI reforms designed to prevent abuses. "What we did not have in place which we should have had in place is a compliance program, to assure that the procedures we had were being followed across the field and that our data bases were accurate and up-to-date. We did not have that. We have put that in place," he said.

Despite these assurances, a bipartisan group of lawmakers has introduced legislation aimed at tightening law governing National Security Letters and strengthening legal protections for those affected by them.

The legislation is supported by a number of civil liberties organizations, including the American Civil Liberties Union (ACLU).

"The NSL statute has long been in need of repair. This bill will re-align NSL authorities with the Constitution, and reaffirm that Americans can be both safe and free," said Michelle Richardson is with the ACLU Washington office.

Among other things, the proposed legislation would restore a standard requiring the government to show that a National Security Letter relates to a suspected terrorist or spy, give the recipient of such a letter the right to challenge it, and limit the duration of the government's right to prevent a recipient from disclosing that they received the letter.