Washington ? FBI Director Robert Mueller voiced little anxiety about upcoming congressional hearings designed to probe how the nation’s federal law enforcement and intelligence services failed to anticipate the Sept. 11 terrorist assaults.

“The attack happened, and we have the responsibility of preventing such attacks,” he told reporters during a briefing Friday, adding that he welcomed suggested improvements from Congress, state and local law enforcement and others.

“The hardest thing for all of us is to think that another such attack will occur and we have not done everything possible we can, overturned every stone, to try to prevent the next attack,” Mueller said. “We want to do a better job of connecting the dots so that this does not happen again.”

That said, he and other FBI officials have noted the difficulty in detecting 19 hijackers who kept such a low profile that they aroused virtually no suspicions.

Criticism of the FBI which hit a crescendo last year with the embarrassing revelations of a spy burrowed deeply within the bureau, lost weapons and computers, and bungled document handling that delayed the execution of Oklahoma City bomber Timothy McVeigh tapered off after Sept. 11.

The bureau’s inability to detect and thwart the plot responsible for the worst act of domestic terrorism will be probed, however, in the coming months by the Senate Judiciary Committee.

September 11 has made reform of the FBI all the more pressing, Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., said as he unveiled legislation with Sen. Charles Grassley, R-Iowa, that would require the FBI to define its missions, improve its information management and tighten internal security.

“This is not a case of chasing the Dillinger gang across mid-America,” Leahy said at a Capitol news conference. “This is an era where we face constant threats, new threats, threats that are unimaginable.”

Grassley, a longtime FBI critic, said the bureau’s “style-over-substance approach” must change. “The FBI must do its main job, and that is preventing terrorism.”

He did express support, however, for Mueller, who took the FBI’s helm just one week before the deadly hijackings and has moved swiftly to reorganize bureau management, update the antiquated case management system and put counterterrorism at the forefront.

“He’s doing the best he can in something that nobody anticipated,” Grassley said.

Mueller warned that Americans should remain on high alert.

“I think we’re still at a high level of risk for terrorist attack,” he said at the briefing with reporters. “There are still those out there who have both the capability and the willingness to attack us in a variety of ways.”