He calls himself Abu Maysara al Iraqi, or father of Maysara the Iraqi, and he's a master at being everywhere and nowhere in the virtual world, constantly switching his online accounts and taking advantage of new technologies to issue his communiques to the world.

American Internet sleuths know next to nothing about him, whether Abu Maysara is his real name, whether he's an Iraqi or even whether he's in Iraq. What is clear is that he is one of the most important sources of information from the country's insurgency, getting his message out through the Internet, and U.S. authorities are trying to silence him.

His updates, terse and businesslike, are released several times a week on radical Islamic Web sites. Acting as a spokesman for Abu Musab Zarqawi, the most wanted guerrilla leader in Iraq, he variously reports attacks on U.S. soldiers and killings of hostages. His words and images reach millions of people when they open their newspapers, turn on their TVs or go online in search of news.

"There's no way of stopping it anymore," said Evan F. Kohlmann, a counterterrorism consultant. "It's extremely frustrating. They can send out quality videos to millions of people uncensored."

As the 2004 presidential election approaches, the Bush administration finds itself in a propaganda war, trying to promote a picture of security and progress in Iraq.

But Abu Maysara's Internet communiques convey another image. Abu Maysara declared in a Sept. 19 posting that he issues his reports so that his perspective "does not become lost in the media blackout that America imposes in order to deceive its people and its allies."

Rita Katz, director of the SITE Institute, a nonprofit group in the District that monitors terrorist sites for the federal government, said she believed that without the Internet, Zarqawi's group would not be as revered as it is today. "Zarqawi has progressed tremendously over the past two years," she said, "and I think that the Internet contributes very much to his reputation."

A Tool of Radicals

The Internet, which was created in the 1960s as a communications network that could survive a Soviet nuclear attack, has emerged as a prime tool of Islamic radicals. They use its anonymity to coordinate operations secretly and to get their message to the public sphere with little fear of detection.

Half a dozen federal agencies have assigned teams to monitor sites that carry postings from Abu Maysara and other radicals. The Justice Department has tried, with limited success, to use the authority of the Patriot Act, passed in the aftermath of the Sept. 11, 2001, attacks, to shut down Internet sites that carry such postings, on grounds that they incite violence.

The government's aggressive pursuit of Web hosting services, as well as the people who post the material on them, has led civil liberties groups to protest that security initiatives are impinging on free speech.

Another problem is that U.S. legal authority stops at the borders. Many of the sites with the target postings are located in other countries, so U.S. officials must depend on the good will of foreign governments to shut down the sites.

Radical groups have also used the Internet to research potential targets, communicate with each other, plan attacks and raise money. After the 2001 attacks in the United States, federal agents found a lengthy electronic trail. They believe that the hijackers coordinated their movements via e-mail, booked their tickets online and used the network to research such subjects as how to spread pesticides by air.

Peter Bardazzi, director of new media development at New York University, contends that the Internet has allowed terrorists to wage psychological warfare as never before, because they have direct control over shaping their own image and that of their foes.

The beheading videos, for instance, are set up "like a stage," he said. "They are trying to inspire followers but also to humiliate the enemy."

Bardazzi added that he believed the videos were changing popular sentiment about the war in Iraq the same way the images of fighting during the Vietnam War affected public opinion.

Today, nearly every active guerrilla group has an online presence, spread across hundreds of Web sites, according to Gabriel Weimann, who was until recently a fellow at the federally funded U.S. Institute of Peace.

Many of the sites are as slick as those of Fortune 500 corporations. One radical Islamic site displays pictures of President Bush and his main ally in the war, British Prime Minister Tony Blair, with stitches covering their bodies and promises that this scene will be "coming soon." It also urges boycotts of Coca-Cola Co., Nike Inc. and other U.S. corporations with an extensive presence overseas.

An Undetectable Hideout

With more than 1 billion linked computers, the Internet is an ideal hiding place for underground groups. The technology allows users to mask their identities and change them on a whim by throwing away old e-mail accounts and creating new ones.

Location can be hidden by hopping virtually from computer to computer. A message that appears to come from Australia, for instance, may actually come from someone who has accessed the Australian computer by going through the Netherlands via South Korea after originating in Jordan.

Secrecy is helped by the proliferation of free e-mail and Web hosting services that require users to give a minimal amount of information, which is rarely verified.

Terrorism investigators first began to see the name Abu Maysara in January in the Muntada Al-Ansar and Islah chat rooms, which are password-protected and only available to users who pass a background screening interview with a battery of questions including such things as whether they have ever attacked U.S. citizens and whether they have been imprisoned. Abu Maysara confirmed Zarqawi's presence in Iraq and took credit for several attacks on U.S. and other coalition troops.

Investigators believe it was the Ansar site that, in May, carried the first video depicting the beheading of Nicholas Berg, the young Pennsylvania businessman who was in the country looking to make money repairing radio towers.

For months, U.S. agents had been chasing the site, which was once based in Alexandria. By the time Berg was killed, the site had been moved to Malaysia.

Authorities managed to get it shut down again, but only temporarily. Ansar recently reappeared on a server in West Lafayette, Ind.

In June, the government's legal assaults suffered a setback when a computer programmer in Idaho was acquitted of a criminal charge that he had run a Web site that included edicts calling for violence. Deputy Defense Secretary Paul D. Wolfowitz has told Congress that the government will continue to pursue operators of such sites.

Feeling the pressure, Abu Maysara has been experimenting with new ways to get his message out effectively, investigators said. The problem with the Berg posting was that it was relatively easy for authorities to stop its distribution because it was only placed on one or two Web sites. E-mailing the video was out of the question because the file was too large.

Abu Maysara could not use the free Web hosting services from Yahoo or other companies because those services limited the number of people who can view a file.

But some time last month, Abu Maysara found a silver bullet -- a technology called YouSendIt.

Developed by three Canadian programmers in Silicon Valley in California, it allows senders to create multiple links to a large file so it can be viewed by an unlimited number of people. Users type in their e-mail addresses, upload the file and YouSendIt creates a free, anonymous Web page for them. To distribute videos of the contractors who were kidnapped this month, Abu Maysara created dozens of links using YouSendIt and sent them to chat rooms all over the Internet.

He compressed the files, or made them as small as possible, investigators said, so that they could be copied more quickly. By the time U.S. officials got word of the videos, they had been anonymously copied from computer to computer as fast as a Top 10 music hit would have been during the peak of the music service Napster's operations -- making it impossible to locate, much less destroy, all the copies of the video.

Khalid Shaikh, president of YouSendIt Inc., based in Campbell, Calif., said he created the program to help families trade pictures and videos and to help colleagues at work share files such as multimedia presentations. He said he was surprised and saddened to hear that the technology was being used to spread violent messages.

Shaikh said he had not been contacted by any law enforcement authority regarding the use of his service by terrorists but that he would be eager to help U.S. officials. But, he said, since there are more than 1 million file transfers a day on YouSendIt, it would be impossible for the company to monitor them all.

"It's almost like policing a society," he said. He also said the company's philosophy is to let users monitor each other. The company recently added technology to allow viewers to have a link deleted if they deem it offensive.

Public Participants

The messages from Abu Maysara follow a rigid format. They are always in Arabic and open with a standard greeting such as, "In the name of Allah, Most Gracious, Most Merciful." Then comes the heart of the message, written in flowery language, recounting an attack. Abu Maysara favors ellipses, writes in half-sentences and mixes the details of an incident with religious invocations.

In some chat rooms, Abu Maysara and his collaborators are hailed as heroes. On Sept. 22, after he announced the beheading of Eugene "Jack" Armstrong, 52, a native of Hillsdale, Mich., dozens of people on one Web site thanked him and Zarqawi.

"I love you, I love you, O slaughtering Sheikh!" one person wrote, according to a translation by the SITE Institute. Another said that he would "pray to Allah that He protect you, O Mujahideen of Iraq."

On occasion, chat room participants act like fans and pummel Abu Maysara with questions about his background and how and why he joined the group, said Kohlmann, the terrorism expert.

All they get from Abu Maysara is silence. "He doesn't respond to requests for information. He's never replied to any message. He's almost like a robot," Kohlmann said. "He never gets involved in the discussion. He never explains himself."

Still, many of the people who read Abu Maysara's postings follow up with advice about new attack strategies. In the past, Katz said, Abu Maysara seemed to ignore them, but in recent days Katz noticed something that was either a remarkable coincidence or a change in Abu Maysara's habits.

In a posting on Sept. 22 at 3:46 a.m., time zone unspecified, a person calling himself Nimr suggested that the group make Kenneth Bigley, the 62-year-old British citizen who had been kidnapped with Armstrong and another American, Jack Hensley, 48, of Marietta, Ga., beg for his life to the "tyrant."

A few days later, a clip appeared on the Web site with a distraught Bigley pleading to British Prime Minister Tony Blair to save him.

Staff writer Nora Boustany and researchers Robert E. Thomason in Washington and Richard Drezen in New York contributed to this report.

This radical Islamic chat room keeps an archive of messages from Abu Maysara, who acts as a spokesman for a key Iraqi insurgent. Abu Maysara's name first appeared on the Ansar site, which investigators believe was the first radical chat room to show Nicholas Berg's beheading.