"You have done what so many said could not be done: You have enhanced the freedom of Americans at precisely the time when our freedom has been under its greatest attack," Ashcroft said in prepared remarks.
Ashcroft has said he will step down as soon as his nominated successor, White House counsel Alberto Gonzales, is confirmed by the Senate.
The attorney general noted there had been no repeat of the Sept. 11, 2001, terror attacks.
Ashcroft also said "violent crime has been driven down to its lowest rate in 30 years."
"In the past three years alone, from 2001 to 2003, we have driven down the overall rate of violent crime by 27 percent beyond the previous three-year period," he said.
In addition, the Corporate Fraud Task Force "has charged more than 900 violators in more than 400 cases since its inception," Ashcroft said.
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