Terrorists increasingly are using the Internet as a means of communication both with each other and the rest of the world. By now, nearly everyone has seen at least some images from propaganda videos published on terrorist sites and rebroadcast on the world's news networks. Western governments have intensified surveillance of such sites but their prosecution of site operators is hampered by concerns over civil liberties, the Internet's inherent anonymity, and other factors.

The Internet is a powerful tool for terrorists, who use online message boards and chat rooms to share information, coordinate attacks, spread propaganda, raise funds, and recruit, experts say. According to Haifa University's Gabriel Weimann, whose research on the subject is widely cited, the number of terrorist sites increased exponentially over the last decade--from less than 100 to more than 4,800 two years ago. The numbers can be somewhat misleading, however. In the case of al-Qaeda, hundreds of sister sites have been promulgated but only a handful are considered active, experts say. Nonetheless, analysts do see a clear proliferation trend.

Terrorist websites can serve as virtual training grounds, offering tutorials on building bombs, firing surface-to-air missiles, shooting at U.S. soldiers, and sneaking into Iraq from abroad. Terrorist sites also host messages and propaganda videos which help to raise morale and further the expansion of recruitment and fundraising networks. Al-Qaeda's media arm, As-Sahab, is among the most visible. But an entire network of jihadist media outfits has sprung up in recent years, according to a March 2008 study by Daniel Kimmage of Radio Free Europe/Radio Liberty. "The 'original' al-Qaeda led by Osama bin Laden accounts for a mere fraction of jihadist media production," Kimmage writes.

Defining a terrorist website is as contentious as defining terrorism. Pentagon analysts testifying before Congress have said that they monitor some five thousand jihadi websites, though they closely watch a small number of these—less than one hundred—that are deemed the most hostile.

Terrorist sites include the official sites of designated terrorist organizations, as well as the sites of supporters, sympathizers, and fans, says Weimann. But when websites with no formal terrorist affiliation contain sympathetic sentiments to the political aims of a terrorist group, the definition becomes murky. Hoax sites can also prove a troublesome red herring for monitors of terrorist sites. For instance, in recent years a number of sites sympathetic to the Taliban have proliferated on the web. Frequent site outages, however, make it difficult to track their content and sentiment.

Perhaps the most effective way in which terrorists use the Internet is the spread of propaganda. Abu Musab al-Zarqawi's al-Qaeda cell in Iraq has proven particularly adept in its use of the web, garnering attention by posting footage of roadside bombings, the decapitation of American hostage Nick Berg, and kidnapped Egyptian and Algerian diplomats prior to their execution.

In Iraq, experts say terrorist propaganda videos are viewed by a large portion of society, not just those who sympathize with terrorists and insurgents. In addition to being posted online, the videos are said to be sold in Baghdad video shops, hidden behind the counter along with pornography. Evan Kohlmann, an expert in terrorists' use of the Internet, points out that propaganda films are not exclusively made in the Middle East; groups from Bosnia, Afghanistan, and Chechnya have also produced videos. Nor are videos the only form of propaganda. Some jihadi websites have even offered video games in which users as young as seven can pretend to be holy warriors killing U.S. soldiers.

"The greatest advantage [of the Internet] is stealth," says John Arquilla, professor of defense analysis at the Naval Postgraduate School. "[Terrorists] swim in an ocean of bits and bytes." Terrorists have developed sophisticated encryption tools and creative techniques that make the Internet an efficient and relatively secure means of correspondence. These include steganography, a technique used to hide messages in graphic files, and "dead dropping": transmitting information through saved email drafts in an online email account accessible to anyone with the password.

The Internet also provides a global pool of potential recruits and donors. Online terrorist fundraising has become so commonplace that some organizations are able to accept donations via the popular online payment service PayPal.

Yet some terrorism experts say while the Internet has proven effective at spreading ideology, its use as a planning and tool operational tool is minimal. For instance, the terrorists who attacked Mumbai in November 2008 could never have carried out their strike if they hadn't received actual training in a physical camp, says terrorism expert Peter Bergen. "We talk about the Internet being important for terrorism, which I think is ridiculous. The people who did the Mumbai attack didn't sit around reading about how to do attacks on the Internet. They actually went to a training camp in Muzaffarabad for several months."

Cyberterrorism is typically defined as the use of the Internet as a vehicle through which to launch an attack. Terrorists could conceivably hack into electrical grids and security systems, or perhaps distribute a powerful computer virus. "Al-Qaeda operatives are known to have taken training in hacking techniques," Arquilla says, but the likelihood of such a cyber attack seems fairly remote. That said, Western governments have accused state and nonstate actors of infiltrating secure networks, including an alleged breach of a Pentagon system by Chinese hackers in June 2007.

Kohlmann suggests the established definition of cyberterrorism needs to be broadened. He says any application of terrorism on the Internet should be considered cyberterrorism. "There's no distinction between the online [terrorist] community and the real [terrorist] community." As evidence, Kohlmann recounts one extreme instance in which the Iraqi insurgent group Army of the Victorious Sect held a contest to help design the group's new website. According to Kohlmann, the prize for the winning designer was the opportunity to, with the click of a mouse, remotely fire three rockets at a U.S. military base in Iraq.

Among the most infamous figures to emerge from the world of online terrorism in recent years is "Irhaby 007" ("Terrorist 007"). As a SITE Institute profile explains, Irhaby 007 was celebrated by other online terrorists for his hacking prowess and his ability to securely distribute information. With his assistance, terrorist organizations around the globe were able to expand the reach of their message. Irhaby 007 passed this knowledge along to other online jihadis through web postings such as his "Seminar for Hacking Websites," creating a network of technology-savvy terrorist disciples. In October 2005, Scotland Yard officers in West London arrested 22-year-old Younis Tsouli, whom they later identified as Irhaby 007. Tsouli is awaiting trial on charges that include conspiracy to murder and terrorist fundraising.

Another prominent online terrorist is Abu Maysarah al-Iraqi, who has served as the media representative for al-Qaeda in Iraq leader Zarqawi. Al-Iraqi goes online to claim responsibility for acts of terrorism, post propaganda videos, and issue statements on behalf of Zarqawi, though experts say it is unclear whether al-Iraqi is just one person or several using the same name. Al-Iraqi's current whereabouts are unclear; a March 2007 report from the Iraqi news agency Quds Press, translated by the BBC, indicated that the al-Qaeda spokesman had been released from a Mosul prison.

In November 2008, Ali Hamza Ahmad Suleiman al-Bahlul, al-Qaeda's digital media director, was convicted by a U.S. military tribunal. He is serving a life sentence at Guantanamo Bay.

There is some debate within the counterterrorism community about how to combat terrorist sites. "The knee-jerk reaction is if you see a terrorist site you shut it down," Kohlmann says, but doing so can cause investigators to miss out on a wealth of valuable information. "You can see who's posting what and who's paying for it," says Michael Kern, formerly a senior analyst at the SITE Institute, a Washington-based terrorist-tracking group. For instance, German officials monitoring online chatter issued early warnings prior to the Madrid train bombings in March 2004.

Shutting down a terrorist website is just a temporary disruption. To truly stop a terrorist site, experts say, the webmaster must be stopped. The ability of the U.S. National Security Agency to monitor such individuals inside the United States has been the subject of a heated political and legal debate. The United States has tried to prosecute webmasters who run terrorist websites in the West, but has run into opposition from advocates of free speech. "Sites that tell the terrorist side of the story go right up to the brink of civil liberties," Arquilla says. Sami Omar al-Hussayen, a Saudi Arabian graduate student at the University of Idaho, was charged by U.S. officials with supporting terrorism because he served as a webmaster for several Islamic groups whose sites linked to organizations praising terrorist attacks in Chechnya and Israel. Al-Hussayen was acquitted of all terrorism charges by a federal court in June 2004 under the First Amendment. Two months later, Babar Ahmad, a 31-year-old, British-born son of Pakistani immigrants, was arrested in London under a U.S. warrant.

Another approach officials have taken is to create phony terrorist websites. These can spread disinformation, such as instructions for building a bomb that will explode prematurely and kill its maker or false intelligence about the location of U.S. forces in Iraq, intended to lead terrorist fighters into a trap. This tactic must be used sparingly, says Kohlmann, or else officials risk "poisoning a golden pot [of information]" about how terrorists operate.