Rita Katz is tiny and dark, with volatile brown eyes, and when she is nervous or excited she can’t sit still. She speaks in torrents, ten minutes at a stretch. Everybody who works in intelligence calls her Rita, even people who don’t know her well. She sometimes telephones people she hasn’t met—important people in the government—to tell them things that she thinks they ought to know. She keeps copies of letters from officials whose investigations into terrorism she has assisted. “You and your staff . . . were invaluable additions to the investigative team,” the special agent in charge of the F.B.I.’s Salt Lake City Division wrote; the Assistant U.S. Attorney in Boise said, “You are a rare and extraordinary gem that has appeared too infrequently throughout the course of history.” The letters come in handy, she told me, when she meets with skepticism or lack of interest; they are her establishment bona fides.

Katz, who was born in Iraq and speaks fluent Arabic, spends hours each day monitoring the password-protected online chat rooms in which Islamic terrorists discuss politics and trade tips: how to disperse botulinum toxin or transfer funds, which suicide vests work best. Occasionally, a chat-room member will announce that he is turning in his user name and password and going to Iraq to become a martyr, a shaheed. Several weeks later, his friends will post a report of the young man blowing himself up. Katz usually logs on at six in the morning. When she has guests for dinner, she leaves a laptop open on the kitchen counter, so she can check for updates. “It is completely addicting,” she says. “You wake up thinking, I’ve been offline for seven hours, but the terrorists have been making plans.”

Traditionally, intelligence has been filtered through government agencies, such as the C.I.A. and the N.S.A., which gather raw data and analyze it, and the government decides who sees the product of their work and when. Katz, who is the head of an organization called the Search for International Terrorist Entities, or SITE Institute, has made it her business to upset that monopoly. She and her researchers mine online sources for intelligence, which her staff translates and sends out by e-mail to a list of about a hundred subscribers.

Katz’s client list includes people in the government who are presumably frustrated by how long it takes to get information through official channels; it also includes people in corporate security and in the media, who rarely get much useful material from the C.I.A. She has worked with prosecutors on more than a dozen terrorism investigations, and many American officers in Iraq rely on Katz’s e-mails to, for example, brief their troops on the designs for explosives that are passed around terrorist Web sites. “You’re thrown into Baghdad, and there are a million different groups out there you’ve never heard of claiming responsibility for attacks,” Robert Worth, a Times reporter who used Katz’s service during the eighteen months he spent in Iraq, told me. “Rita really knows what she’s talking about—who’s responsible for attacks, what’s a legitimate terrorist organization and what’s not.” Because many reporters rebroadcast her information, it can reach the public before people in the government have had a chance to evaluate it; her organization’s work is cited in the Times and the Washington Post about twice a month.

Katz has many critics, who believe that she is giving terrorists a bigger platform than they would otherwise have, and that the certainty and obsession that make her a dedicated archivist also make her too eager to find plots where they don’t exist; she publicized a manual for using botulinum in terror attacks, for example, which experts later concluded was not linked to any serious threat. It’s possible that her immersion in the world of terrorism has removed whatever skepticism or doubts she may have had. “Much as Al Jazeera underplays terrorist threats, the SITE Institute at times overhypes them,” Michael Scheuer, the former head of the C.I.A.’s bin Laden unit, said.

More fundamentally, some people involved in counterterrorism do not think that a private group with limited resources can do as good or as prudent a job as government agencies can. “Intelligence analysis is a set of skills that you learn, not just something that anyone can walk in off the street and pick up,” Steven Aftergood, who monitors the intelligence community for the Federation of American Scientists, told me. Katz, however, pointed out that, for example, the professionals consistently missed signals about Al Qaeda before September 11, 2001, and said that she was simply filling a gap. (A 2004 audit showed that the F.B.I. alone had thousands of hours of untranslated intercepts.) Indeed, Katz has received outsourcing contracts from the government.

Before the September 11, 2001, attacks, the official counterterrorism agencies paid relatively little attention to the jihadis’ online presence. But in the past few years that has changed, in large measure because of changes in the way terror networks operate. “Nearly everything about Al Qaeda that matters is happening online right now,” Peter Bergen, a journalist and terrorism expert, said. Some analysts believe that Al Qaeda today is a model of what is called “leaderless resistance”: self-appointed cells operating with help and inspiration from materials that they find online. Traffic rose dramatically after Abu Musab al-Zarqawi, the head of Al Qaeda in Iraq, posted a video of the beheading of the American contractor Nicholas Berg.

“It’s not as if Al Qaeda were inventing this,” Jessica Stern, a terrorism specialist who served on the National Security Council under President Clinton, said. What’s unique about Islamic terror and the Internet is that there is up-to-the-minute access to what terrorists are thinking. Rita Katz is, in a sense, the natural complement, the engineer of a leaderless counter-resistance to the terrorist groups. “Some people think that she’s a zealot,” Stern said when I asked her about Katz, “but only a zealot would provide this kind of service.”

In March, I visited Katz at her office, on the seventh floor of an old building in a Northeastern city that she refuses to allow reporters to identify in print. She told me to take a train to the city’s main terminal, and then call the office for further directions. By the time I got to SITE’s locked door, which has a black security camera and a plaque bearing the name of a nonexistent business, I half expected to walk into a center full of high-tech equipment, with flashing maps and screens.

The SITE Institute’s office looked like a college newspaper’s. There were three rooms: Katz’s office, dominated by a large conference table; a small room for two translators (more work part time, from home); and what’s called the pit, where several researchers and interns, all in their twenties, sat under a long, eye-level row of mug shots of wanted terrorists—mostly bearded Arab men, with grim, unsmiling glares. There was an air of intense isolation, with everyone focussed on his own projects. It was hard to ignore the office’s youth; Katz told me of a new service she had added that scanned French-language terrorist sites, and that depended on an undergraduate intern who spoke fluent French (Katz has since hired another French-speaker for the service).