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A spike in domestic terrorism and attacks by American citizens directed from overseas are top concerns for police departments across the country, according to a new survey by the Homeland Security Policy Institute at George Washington University.

“Homegrown and foreign-directed jihadi terrorism and radicalization are perceived as a real threat by local law enforcement in the United States,” the report, “Counterterrorism Intelligence: Law Enforcement Perspectives,” says. The survey covered the police intelligence chiefs for the 56 largest cities in the U.S. in advance of the 10-year anniversary of the 9/11 attacks.

The survey identifies 21 homegrown terrorism cases from Sept. 11, 2001 through May of 2009, but 31 cases in the last two years alone -- more than one new case of homegrown terrorism every month.

Speaking at the Intelligence and National Security Alliance in Washington, the chairman of the House Intelligence Committee said the plots are real and credible.

“Yes it's a threat. Yes, we worry about somebody grabbing a gun and then going down some place and doing something awful,” Rep. Mike Rogers, R-Mich. told the forum. “But they will never just do that. In all the cases you have seen, there are indicators leading up to that particular event. They were radicalized in order to get there.”

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After 9/11, the U.S. intelligence community believed there needed to be person-to-person contact for an individual to cross the threshold to violence. Now, it may be possible to do it virtually. Social networking is creating a new generation of digital jihadists.

Homeland Security Secretary Janet Napolitano, also speaking at the event, acknowledged that the web is the driver of radical Islam and the new generation of Al Qaeda 2.0.

“The increasingly savvy use of the Internet, mainstream and social media and information technology by these groups adds an additional layer of complexity to an already complex threat picture,” she said.

The new report also found that the intelligence chiefs believe gaps remain a decade after the attacks. In addition to believing that the U.S. lacks “an adequate understanding of the counterterrorism intelligence enterprise,” the chiefs cited a lack of access to some intelligence products and said in some cases the information lacked detail: it wasn’t shared adequately, or the data was stale or there was just too much of it.

Philip Mudd, a fellow at the liberal New America foundation who worked in the counterterrorism world for more than two decades and served in both the Bush and Obama administrations, told Fox News that marrying the federal, state and local law enforcement systems was not an easy task.

“We need to have standard rules and regulations about if we get something in San Francisco can we give it to Topekam” he said.

Mudd added that Al Qaeda has evolved and homegrown plots are the latest manifestation. He said it is unlikely the number of cases has peaked.

“This is a new art form and it's not even the art form we would have had five or six years ago,” Mudd said referring to intelligence analysis. “This homegrown phenomenon is really only about three or four years old. So it's not just how do we respond after 10 years of the terrorism problem, it's how do we respond to the problem of the Al Qaeda revolution when we're not even dealing with Al Qaeda members anymore.”

National Correspondent Catherine Herridge's bestselling book "The Next Wave: On the Hunt for al Qaeda's American Recruits" was published by Crown on June 21st. It draws on her reporting for Fox News into al-Awlaki and his new generation of homegrown recruits – Al Qaeda 2.0. It is the first book to full investigate al-Awlaki’s American life, his connections to the hijackers, and how the cleric double crossed the FBI after 9/11.