"Little evidence of jihadists in the U.S."

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.

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.