Senators frustrated by scant details on the Bush administration's domestic eavesdropping program on Wednesday rapped FBI Director Robert Mueller for refusing to show how it has curbed terrorist activity in the United States.

Mueller said he was unable to talk about the warrantless spying program because it is classified.

"What assurances can you provide that the program is worthwhile?" asked Senate Judiciary Chairman Arlen Specter, R-Pa. "Have arrests been made? Have terror cells been broken?"

The FBI has briefed congressional intelligence committees on the controversial program, but Mueller said he did not have permission to share that information with other lawmakers — including the judiciary panels that oversee the bureau.

"I am prepared to brief whichever committee, to the extent that I am allowed to," he said.

In his opening remarks, Mueller ticked off a list of FBI cases targeting terror suspects since the 2001 attacks. The included the so-called "Lackawanna Six" who allegedly attended Al Qaeda training camps; an Ohio truck driver who plotted to attack the Brooklyn Bridge; and four men charged with planning to hit synagogues and U.S. and Israeli facilities in the Los Angeles area.

But Mueller did not say if any of the cases resulted from the secret spying program, which was revealed last year. His answer annoyed senators, who said their constitutionally protected oversight was being hampered by the administration's stonewalling.

"When done poorly or without proper safeguards and oversight, data banks do not make us safer, they just further erode Americans' privacy and civil liberties," said Sen. Patrick Leahy, D-Vt., the panel's incoming chairman. He said the administration "has gone to unprecedented lengths to hide its own activities from the public, while at the same time collecting and compiling unprecedented amounts of information about every citizen."

Mueller's appearance marked a long-delayed hearing for senators eager to hear how the FBI has tried to curb violent crime and upgrade its internal case management computer system. Sen. Dianne Feinstein, D-Calif., scolded him for putting violent crime prevention behind public corruption and organized crime cases.

"Gangs are killing more people in this country than organized crime ever did," Feinstein said. "And that's just a fact."

Responding, Mueller said that if the FBI didn't go after crooked public officials and organized crime cases, "it will not be investigated." He called public corruption cases the FBI's "top criminal investigative priority," noting that more than 1,000 government employees, including hundreds of police officers, have been convicted over the last two years with the FBI's help.