PHILADELPHIA - It was a major terrorism case with a flaw. Prosecutors strongly suspected the defendants had been visiting terrorist Web sites, yet they could not prove it.

The sites had disappeared from the Internet.

But Evan Kohlmann had them, stored on the computer in his studio apartment here.

In part because of his testimony, the defendants were all convicted last month in federal court in Virginia on charges of supporting terrorism.

Kohlmann, 25 and finishing law school, represents an eclectic new breed of terrorism expert that has surfaced since Sept. 11, 2001 -- young, Internet-savvy and schooled outside the intelligence establishment.

He has written articles about everything from regional nesting grounds for terrorists in Europe to the legal and investigative loopholes in cyber-terrorism cases.

He is publishing a book this spring that has won praise from, among others, Richard Clarke, the former counterterrorism chief in the Clinton and Bush administrations.

Clarke has said that the Bush White House did not pay enough attention to the terrorist threat during its first months in office.

And Kohlmann has joined the talking-head circuit, appearing regularly on TV networks, using debating skills he says he developed in high school.

He is building a career -- as his Web site describes it -- as an international terrorism consultant.

Once, only a handful of bookish academics or former government officials focused on terrorism. In the world of public policy, it was a backwater.

But since Sept. 11, 2001, experts have been in high demand.

They're providing security-related information to companies. They're helping retool the government's response to terrorism. They're feeding 24-hour news junkies.

Today, experts are just as apt to be former journalists or entrepreneurs with high-tech backgrounds as they are policy wonks looking to cash in -- although there are plenty of those, too.

For prosecutors more familiar with bank robbers than jihadis, such terrorism expertise can be invaluable.

"Most people, including myself, have never heard of any of these (experts)," said Charles Gorder, an assistant U.S. attorney in Portland, Ore., who hired Kohlmann last year as an expert in the case of a group of alleged terrorists known as the Portland Seven.

Questions are being raised about the credentials and motives of some of the experts.

The defense lawyers in a major federal terrorism trial that began last week in Idaho are seeking to limit the testimony of government witnesses whom they describe in court papers as "self-proclaimed experts who ... have attempted to capitalize on the terrorism 'cottage industry.'"

Their identities are under court seal; the government declined to comment.

A Washington-area expert, Rita Katz, has been named in three lawsuits in connection with her work helping the government investigate Islamic charities in northern Virginia.

In two of the suits, targets of the investigation say they were defamed in a "60 Minutes" TV broadcast in which Katz discussed her work aiding the United States in several terrorism-related investigations by sneaking undercover into mosques linked to radicals.

Although Katz wore a fake nose and wig to hide her identity on the TV program, the lawsuit revealed her identity.

Critics argue that Katz is biased toward Israel.

By her own account in a memoir published last year, she is an Iraqi-born Jew whose father was arrested and executed in the early days of the Saddam Hussein government on trumped-up charges that he was an Israeli spy.

Katz now has her own Washington-based terrorism consultancy. She has worked with federal investigators in terrorism cases and is cited in Clarke's book, "Against All Enemies," as helping to provide information to the government on the al-Qaida network.

She also has been a consultant in a controversial $1 trillion wrongful-death suit seeking to hold Saudi government and business interests accountable for the Sept. 11 attacks.

"She does serious research, and a lot of that research is good and accurate," said one of her peers, who requested anonymity.

"But she fits everything into a mold -- that there is a Muslim terrorist under everybody's bed."

Katz says she has been the victim of a smear campaign by a minority of activists and that her record speaks for itself.

Longer experience and a more traditional background in terrorism studies have been no guarantee of future performance when it comes to analyzing terrorism trends.

Two months before the Sept. 11 attacks, a former State Department counterterrorism official, Larry Johnson, wrote an opinion piece in the New York Times saying that the terrorism threat facing the country had been greatly exaggerated.

But his credibility doesn't seem to have suffered; he is still widely quoted.

"I got a slew of hate mail," Johnson said. But he stands by one of his main points: that terrorism is being used to justify unwarranted expenditures by the government.

The point of entry for many of the new terrorism experts has been technology. Just as terrorist groups have come to use the Internet as a recruitment and propaganda tool, an understanding of technology has become a way of fighting back.

Ben Venzke runs a company called IntelCenter, which specializes in providing risk assessments and other terrorism-related information to government agencies and businesses.

Among its products: "quality video" of al-Qaida members and intelligence updates that can be instantly messaged to a pager or personal digital assistant.

"At this point, it is hard to find a government agency we don't have some level of involvement with," said Venzke, 30, who has never worked in government, has a background in journalism and is a regular contributor to CNN.

And there is Kohlmann. He said he had had an interest in fringe groups for as long has he could remember, cutting his teeth on books about the John Birch Society in high school.

"I started when I was 18," he said. "I have done this for a quarter of my life."

During his freshman year at Georgetown University, he got an internship with a counterterrorism think tank called the Investigative Project, which included Katz and is run by Steven Emerson.

A former investigative journalist, Emerson is a terrorism sleuth whose 1994 documentary "Terrorists Among Us: Jihad in America" was one of the first in-depth looks at militant Islamic groups operating on U.S. soil.

Kohlmann traveled undercover to check out alleged terrorist groups in Europe, and helped his more senior colleagues brief government officials.

An FBI agent the company was working with dubbed him "the Doogie Howser of terrorism," a reference to the TV series about a teenage doctor.

He began writing articles that appeared in scholarly journals.

A piece that he published -- in the spring of 2000 -- about the growth of Internet sites sponsored by Islamic militants exhorting violence concluded: "Unless decisive action is taken soon, this building fanaticism will certainly manifest itself beyond mere online propaganda into an outburst of uncontrolled violence."

On the morning of Sept. 11, he was sitting in one of his first-year law school classes at the University of Pennsylvania when news of the attacks broke.

"I turned to someone next to me," he recalled, "and I said, 'This is Osama bin Laden, and I've got to go.'"

Ever since, he has juggled his studies with his fledgling consulting business.

Among other work, he has served as a witness in two federal terrorism prosecutions and briefed government anti-terrorism task forces.

He spends hours mining the Internet from a computer in his apartment, which is decorated with fliers for al-Qaida camps and wanted posters for terrorists.

The government turned to him in a trial in Virginia in February, in which the defendants had been charged with aiding the Pakistan-based Lashkar-e-Taiba, which the State Department had designated a terrorist group.

Among the evidence was e-mail traffic in which the defendants were enthusiastically commenting on a number of suspicious Web sites.

The government enlisted Kohlmann to track down the vanished sites, and he hit pay dirt when he found them in his computer archive.

Among other scenes, the sites featured anti-American propaganda such as a graphic showing the New York skyline in flames, two years before the Sept. 11 attacks.

The images provided some powerful evidence. The judge hearing the case mentioned portions of Kohlmann's testimony in her opinion finding the men guilty. Sentencing is set for June.

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