NEW YORK (CNN) -- Six years of investigations and prosecutions have turned up little evidence of Islamic jihadists at work in the United States, according to a study released Monday.

Zacarias Moussaoui was convicted in 2006 of conspiring to kill Americans on September 11, 2001.

The study, conducted by New York University's Center on Law and Security, tracked 510 cases billed as terrorism-related when arrests were made.

But it found only 158 of those people arrested since al Qaeda's September 11, 2001, attacks were prosecuted for terrorism.

In a statement issued Monday afternoon, the Justice Department said the report "reflects a serious misunderstanding" of anti-terrorism efforts and includes "wildly inaccurate" statistics.

The study found only four people -- including confessed al Qaeda operative Zacarias Moussaoui and "shoe bomber" Richard Reid -- were convicted of planning attacks within the United States.

"The vast majority of cases turn out to include no link to terrorism once they go to court," the report found. The analysis "suggests the presence of few, if any, prevalent terrorist threats currently within the U.S."

The report questioned the usefulness of the anti-terrorist USA Patriot Act, passed after the September 11, 2001, attacks, finding prosecutors relied primarily on previous laws.

"Although we are just beginning to discern the true extent and manner in which the administration has used the sweeping investigative powers granted by the Patriot Act, the record indicates that the criminal law provides an adequate tool set for trying suspected terrorists," the report stated.

In his 2006 State of the Union address President Bush urged Congress to reauthorize the Patriot Act.

"We now know that two of the hijackers in the United States placed telephone calls to al Qaeda operatives overseas. But we did not know about their plans until it was too late," Bush said.

"So to prevent another attack --- based on authority given to me by the Constitution and by statute -- I have authorized a terrorist surveillance program to aggressively pursue the international communications of suspected al Qaeda operatives and affiliates to and from America."

Receiving applause, Bush said, "If there are people inside our country who are talking with al Qaeda, we want to know about it, because we will not sit back and wait to be hit again."

The Justice Department disputed the law center's figures in Monday's study, saying more than the 62 people who the study cited have been convicted of terrorism-related offenses since the September 11, 2001, attacks.

But the law center said most of those 62 cases involved people planning attacks overseas, not inside the United States.

And looking at possible attacks worldwide, just 7 percent of those arrested in what authorities called terrorism-related cases have been convicted of terrorism charges or providing material support for terrorism, the report said.

Justice Department spokesman Dean Boyd said that as of late September prosecutors had won 312 convictions in "terrorism and terrorism-related cases with an international nexus" since the 2001 attacks. And Justice Department statistics found far fewer defendants had been charged with terrorism offenses between September 11, 2006, and September 11, 2007, than the 109 the report asserted.

"Our efforts to prevent terrorist attacks have moved our focus to the preparatory stages of terrorist planning and to those who would support and actively encourage such activity," Boyd said.

He added, "We have used other criminal charges that apply to the facts of each case to disrupt terrorist activity before it occurs."

In many cases, prosecutors used criminal conspiracy charges to obtain convictions against suspects like Jose Padilla, the accused al Qaeda operative found guilty in August of plotting to support overseas terrorism.

The Bush administration jailed Padilla without charges for nearly four years on allegations that he planned to set off a radioactive "dirty bomb" in the United States, but eventually tried him on unrelated charges.

The report praised what it called the increased effectiveness of the FBI, which it credited with breaking up plots to bomb fuel pipelines at New York's John F. Kennedy International Airport and attack an Army post at Fort Dix, New Jersey.

But it questioned the use of what it called "preventive arrests" by federal agents to disrupt plots rather than let agents continue watching suspects and gathering more evidence. E-mail to a friend

CNN's Kevin Bohn contributed to this report.

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