The Patriot Act has been instrumental in the prosecution of hundreds of cases and is "Al Qaeda's worst nightmare," according to Attorney General John Ashcroft (search).

Ashcroft's comments came Tuesday at the release of a 29-page report that is part of a Bush administration effort to discourage Congress from weakening a law that critics say threatens civil liberties by giving authorities more latitude to spy on people. Key sections of the law expire at the end of 2005.

"The Patriot Act is Al Qaeda's worst nightmare when it comes to disrupting and disabling their operations here in America," Ashcroft said.

The report says that in the period starting with the Sept. 11, 2001, terror attacks and ending May 5, Justice Department (search) terrorism investigations resulted in charges against 310 people, with 179 convictions or guilty pleas. The Patriot Act, Justice officials say, was instrumental in these cases.

But some Democrats are not convinced about the legislation's success and want Congress to ask more questions.

"The attorney general's report is no substitute for thoroughgoing congressional oversight. When the attorney general and I spoke this morning I encouraged his department to answer the questions I and others have sent him," Vermont Sen. Pat Leahy said on Tuesday.

Leahy, the ranking Democratic member on the Senate Judiciary Committee (search), asked why the Justice Department has not been able to prosecute successfully anyone over the Sept. 11 attacks and how lack of cooperation and intelligence-sharing failures caused a German court to acquit two Sept. 11 co-conspirators.

"Why are we deporting suspected terrorists like Nabil al-Marabh in lieu of prosecution? These are the questions that could be answered if the Congress really carried out its oversight role," he said.

But Ashcroft, appearing at a news conference with House Judiciary Committee (search) Chairman James Sensenbrenner, R-Wis., offered a different perspective. He said the report provides "a mountain of evidence that the Patriot Act continues to save lives."

Among the specific examples:

— The Patriot Act allowed intelligence agents to share with FBI criminal investigators evidence that an anonymous letter sent to the FBI had come from an individual with Al Qaeda (search) ties. That letter began the investigation into an alleged terror cell in Lackawanna, N.Y., that has resulted in six guilty pleas.

— That same information-sharing authority was used against members of an alleged terror cell in Portland, Ore., that an undercover informant said was preparing for possible attacks against Jewish schools or synagogues. Continued surveillance under the Patriot Act of one suspect led to six others, who likely would have scattered or fled if the first suspect had been arrested right away.

— Terror financing provisions of the law were used in numerous cases, including charges against a member of the Revolutionary Armed Forces of Colombia, or FARC, on charges of being an unlicensed money transmitter. The same authority has been used to prosecute people illegally sending money to Iraq, Yemen, the United Arab Emirates and India.

Powers permitted under the Patriot Act have also been used in investigations involving potential school bomb attacks, computer hackers, child pornography, violent fugitives and illegal weapons sales. In one case, Patriot Act electronic communications authorities allowed law enforcement agencies to identify a person who had sent 200 threatening letters laced with white powder in Lafayette, La., the Justice Department said.

The report did not say whether the FBI had used its authority to obtain library or bookstore records. That information is classified, but Ashcroft last year issued a declassified statement saying that, up to that point, the power had not been used.

House Republican leaders fought back an amendment last week that would have barred the FBI from using Patriot Act authority to obtain library and bookstore records. That was the same week that Justice Department officials wrote Sensenbrenner that investigators had confirmed that "a member of a terrorist group closely affiliated with Al Qaeda" had used Internet services at a public library in winter and spring of 2004 "to communicate with his confederates." No other details were offered.

Rep. John Conyers of Michigan, ranking Democrat on the House Judiciary Committee, accused the department of selectively releasing information about the Patriot Act and refusing to address civil liberties concerns.

"Coupled with the department's consistent record of exaggerating their record about terrorism, this entire report is suspect," Conyers said.

Sensenbrenner said opponents were also guilty of being selective in information they use to undermine the law.

"The people who criticize the Patriot Act cherry-pick their contentions the same way," he said.

Fox News' Molly Hooper and The Associated Press contributed to this report.