One afternoon this spring, a half-dozen young computer engineers sat in the headquarters of Prolexic, an Internet-security company in Hollywood, Florida, puzzling over an attack on one of the company’s clients, a penileenhancement business called MensNiche.com. The engineers, gathered in the company’s network operations center, or noc, on the fourth floor of a new office building, were monitoring Internet traffic on fifty-inch wall-mounted screens. Anna Claiborne, one of the company’s senior network engineers, wandered into the noc in jeans and a T-shirt. The MensNiche attacker had launched an assault on the company’s Web site at 4 a.m., and Claiborne had spent the night in the office fending it off. “Hence,” she said, “I look like hell today.”

MensNiche’s problems had begun a week earlier, with a flood of fake data requests—what is known as a distributed denial-of-service attack—from computers around the world. Although few, if any, of those computers’ owners knew it, their machines had been hijacked by hackers; they had become what programmers call “zombies,” and had been set loose on MensNiche. The result was akin to what occurs when callers jam the phone lines during a television contest: with so many computers trying to connect, almost none could get through, and the company was losing business.

The first wave of the attack was easily filtered by Prolexic’s automated system. The assailant then disguised his zombies as legitimate Web users, fooling the filters so well that Claiborne refused to tell me how it was done, for fear that others would adopt the same tactic. She spent the night examining the requests one by one as they scrolled by—interrogating each zombie, trying to find a key to the attacker’s strategy.

“He’s clever, and he’s been trying everything,” Claiborne said. “If we ever find out who it is, seriously, I’d be willing to buy a plane ticket, fly over, and punch him in the face.”

Prolexic, which was founded in 2003 by a twenty-seven-year-old college dropout named Barrett Lyon, is a twenty-four-hour, seven-days-a-week operation. An engineer is posted in the noc at all times, to monitor Prolexic’s four data hubs, which are in Phoenix, Vancouver, Miami, and London. The hubs contain powerful computers designed to absorb the brunt of data floods and are, essentially, massive holding pens for zombies. Any data travelling to Prolexic’s clients pass through this hardware. The company, which had revenues of four million dollars in its first year, now has more than eighty customers.

Lyon’s main business is protecting his clients from cyberextortionists, who demand payments from companies in return for leaving them alone. Although Lyon is based in Florida, the attackers he deals with might be in Kazakhstan or China, and they usually don’t work alone.

“It’s an insanely stressful job,” Claiborne told me. “You are the middleman between people who are losing thousands or millions of dollars and somebody who really wants to make that person lose thousands or millions of dollars.” When the monitors’ graphs begin to spike, indicating that an attack is under way, she said, “it’s like looking at the ocean and seeing a wall of water three hundred feet high coming toward you.”

Only a few years ago, online malfeasance was largely the province of either technically adept hackers (or “crackers,” as ill-intentioned hackers are known), who were in it for the thrill or for bragging rights, or novices (called “script kiddies”), who unleashed viruses as pranks. But as the Web’s reach has expanded real-world criminals have discovered its potential. Mobsters and con men, from Africa to Eastern Europe, have gone online. Increasingly, cyberextortionists are tied to gangs that operate in several countries and hide within a labyrinth of anonymous accounts.

“When the attack starts, the ticker starts for that company,” Lyon said. “It’s a mental game that you’ve been playing, and if you make a mistake it causes the whole thing to go down. You are terrified.”

Lyon, as usual, was wearing shorts and flip-flops. He has blond hair and a trim build, with narrow hazel eyes that were framed by dark circles of fatigue. A poster for the 1983 movie “WarGames”—a major influence—hung above his desk, on which were four computer monitors: one for writing program code, one for watching data traffic, one for surfing the Web, and one for chatting with customers. Lyon leaned over and showed me a program that he had created to identify the zombies attacking MensNiche. When he ran it, a list of countries scrolled up the screen: the United States, China, Cambodia, Haiti, even Iraq.

Examining the list of zombie addresses, Lyon picked one and ran a command called a “traceroute.” The program followed the zombie’s path from MensNiche back to a computer called NOCC.ior.navy.mil—part of the United States Navy’s Network Operations Center for the Indian Ocean Region. “Well, that’s great,” he said, laughing. Lyon’s next traceroute found that another zombie was on the Department of Defense’s Military Sealift Command network. The network forces of the United States military had been conscripted in an attack on a Web site for penis enlargement.

Michael Alculumbre’s first communication from the extortionists arrived on a Thursday evening in August, 2004. An e-mail message was sent to him just after 8 p.m. at Protx, an online-payment processing company based in London, where he is the chief executive officer. The subject line read, simply, “Contact us,” and the return address—commerce_protection@yahoo.com—offered no clues to the message’s origin. The note was cordial and succinct, written in stilted English. “Hello,” it began. “We attack your servers for some time. If you want save your business, you should pay 10.000$ bank wire to our bank account. When we receive money, we stop attack immediately. If we will not receive money, we will attack your business 1 month.” The note said that ten thousand dollars would buy Protx a year’s worth of protection. “Think about how much money you lose, while your servers are down. Thanks John Martino.” Alculumbre had never heard of John Martino. He decided to ignore the demand.

Two months later, Alculumbre’s network technician called him at home. He said that customers were complaining that the system was off-line. By the time Alculumbre arrived at the office, the source of the disruption was clear. Thousands of computers were inundating Protx’s Web site with fake data requests. Many of Protx’s legitimate customers received the Internet equivalent of a busy signal—a message saying that the company’s servers weren’t responding.

Every minute that the Web site remained off-line, Protx’s business suffered. As the company’s engineers struggled to contain the attack, another ten-thousand-dollar e-mail demand arrived, this time signed “Tony Martino.” Again, Alculumbre ignored it. He had received a call from an agent of the British National Hi-Tech Crime Unit, which had been monitoring the attack. The agent let him know that paying Martino wasn’t an option; the extortionist would only return. Beyond that advice, there wasn’t much that the N.H.T.C.U. could do to help. By the time Alculumbre’s engineers were able to get the site running, it had been disabled for almost two days.

Alculumbre heard from Tony Martino again the following April, when he received a message offering a thousand-dollar-a-month protection-money payment plan. Before he could respond, an army of up to seventy thousand zombies ripped through Protx’s defenses and knocked its Web site off-line. This time, it took Protx’s engineers three days to fight off the attack.

The company now spends roughly five hundred thousand dollars a year to protect itself—fifty times what Martino had asked for. This includes a hundred-thousand-dollar-a-year security contract with Prolexic. Martino, it turned out, had been targeting Lyon’s clients for months before he hit Protx.

“This is very similar to the pubs and clubs in London forty years ago that used to pay money to not have their premises smashed up,” Mick Deats, the deputy head of the N.H.T.C.U., told me. “It’s just a straight, old-fashioned protection racket, with a completely new method.” The cyberextortionists also make use of an elaborate money-laundering system, Deats said. “They have companies registered all over the place, passing the money through them.”

“I started prosecuting network-attack cases in 1992, and back then it was more the sort of lone hackers,” said Christopher Painter, the deputy chief of the Computer Crime and Intellectual Property Section at the Department of Justice. Today, he says, “you have organized criminal groups that are adopting technical sophistication.”

The most potent weapon for Web gangsters is the botnet. A bot, broadly speaking, is a remote-controlled software program that is installed on a computer without the owner’s knowledge. Hackers use viruses, worms, or automated programs to scan the Internet in search of potential zombies. One recent study found that a new P.C., attached to the Internet without protective software, will on average be infected in about twenty minutes.

In the most common scenario, the bots surreptitiously connect hundreds, or thousands, of zombies to a channel in a chat room. The process is called “herding,” and a herd of zombies is called a botnet. The herder then issues orders to the zombies, telling them to send unsolicited e-mail, steal personal information, or launch attacks. Herders also trade, rent, and sell their zombies. “The botnet is the little engine that makes the evil of the Internet work,” Chris Morrow, a senior network-security engineer at M.C.I., said. “It makes spam work. It makes identity fraud work. It makes extortion, in this case, work.”

Less than five years ago, experts considered a several-thousand-zombie botnet extraordinary. Lyon now regularly faces botnets of fifty thousand zombies or more. According to one study, fifteen per cent of new zombies are from China. A British Internet-security firm, Clearswift, recently predicted that “botnets will, unless matters change dramatically, proliferate to the point where much of the Internet . . . comes to resemble a mosaic of botnets.” Meanwhile, the resources of law enforcement are limited—the N.H.T.C.U., for example, has sixty agents handling everything from child pornography to identity theft.

Extortionists often prefer to target online industries, such as pornography and gambling, that occupy a gray area, and may be reluctant to seek help from law enforcement. Such businesses account for most of Prolexic’s clients. I asked Lyon how he felt about the companies he defended. “Everybody makes a living somehow,” he said. “It’s not my job to worry about how they do it.”

I asked whether that applied to extortionists as well. After a pause, he said, “I guess I’m partial to dot-commers.”

Several weeks later, he called me to say that he’d reconsidered his answer. “The Internet is all about connecting things, communicating and sharing information, bits, pieces of data,” he said. “A denial-of-service attack is the exact opposite of that. It is taking one person’s will and imposing it on a bunch of others.” In any case, Lyon added, his clients now included mainstream businesses—a Japanese game company, foreign-exchange traders, and a multibillion-dollar corporation that wanted to have additional security in the days before its I.P.O.

Lyon first gained a measure of online fame in 2003, with a project called Opte, in which he created a visual map of the entire Internet—its backbone, transfer points, major servers. After reading that a similar project had taken several months to complete, he bet a friend that he could do it in a day, and won. (A gorgeously rendered print of the map—which Lyon licenses free of charge—appeared in a travelling exhibition on the future of design.)

Lyon’s obsessive interest in computer networks began early. In the third grade at a Sacramento, California, private school for learning-disabled children—Prolexic’s name derives from Lyon’s pride in overcoming severe dyslexia—he and a friend hacked a simple computer game. In junior high school, Lyon discovered the Internet, and with a friend, Peter Avalos, he soon founded a company called TheShell.com, which provided accounts to chat-room users. But his grades suffered, and, after high school, he failed a year’s worth of classes at California State University at Chico.

When a friend he met online, Robert Brown, offered Lyon a job at his computer-security company, Network Presence, he quit school and took it. Brown sent him off to secure the network of a large insurance company in the Midwest. Lyon was nineteen and, he said, “I looked thirteen. So I wore a suit every day, and I worked my ass off for those guys.” He burned out after two years—“I didn’t know you had to meter yourself”—and returned to school, this time at California State University at Sacramento. There, Lyon signed up for philosophy classes, dumped his computers in a closet, and joined the rowing team. But he couldn’t get away from computers entirely; he still took assignments from his old employer, and he and Avalos (who graduated from the United States Naval Academy and has recently returned from flying P-3s in Iraq) continued to operate TheShell.com. The company’s clients tended to be advanced Internet users, and this had the effect of bringing the site to the attention of hackers. At one point, Lyon was fighting off several zombie attacks a day.

In August, 2002, Dana Corbo, the C.E.O. of Don Best Sports, called Network Presence for help. Don Best, which is based in Las Vegas, is a kind of Bloomberg for the gambling world, providing betting lines for both real-world and online casinos. The company had ignored an e-mailed extortion demand for two hundred thousand dollars, and it was under attack. Network Presence sent Lyon.

The next day, Lyon and another engineer flew to Las Vegas and helped Don Best’s engineers set up powerful new servers. Lyon’s strategy worked: the attackers gave up. Corbo treated them to a night out in Vegas, with dinner in front of the Bellagio fountains. (He also paid Network Presence a fee.)