As CBS News Correspondent Mark Phillips reports, sometimes, the intent is simply propaganda. Sometimes, deeper in the net, hidden within other sites, is something more sinister: sites advocating violence, perhaps even providing instructions and commands. One site, which pops up and then disappears regularly on servers around the world, shows Osama bin Laden and a map of Manhattan.
It contains "red sites (that) appear to be identifying targets for being attacked," says Neil Doyle, a new kind of private detective.
And what worries the freelance cyber-terrorist buster about the site, one of thousands he's discovered, are the numbers running across the page.
"The repeated sequence at the top there and it's thought that, well, that does match up to a known al Qaeda cryptography method," says Doyle.
Doyle believes the site's code could be a means of command and control.
"As soon as you discover one site, it closes down within a matter of hours," says Doyle. "Sometimes it crops up somewhere else on the Web."
The site may have begun life in an Internet cafe in Pakistan, but was last tracked down to a server in Singapore. Using proxy servers continents away, terrorists can graft their sites anywhere. It's not called the World Wide Web for nothing. In fact, most of the sites are now located right here in the United States.
"The hottest country right now is America," says Doyle. "There's a recent survey which found that 76 percent of the top 25 terrorist Web sites are actually based on American computer systems.
The war on terror on this level comes down to bad-guy computer nerds versus good-guy computer nerds.
"It's quite clear that a number of terrorists certainly in the al Qaeda network are frankly people that we would describe as computer nerds, and they're able to make evolutionary steps to keep one jump ahead of the authorities," says Crispin Black, a former British Intelligence officer.
Some of the sites are simply terror manuals, like one that shows how to build a bomb into a wedding cake.
"It's got some wires that run thru the icing on the cake, so when the knife touches the wires, it completes the circuit and the bomb detonates," says Doyle.
Farfetched? Not really. The instructions on how to trigger bombs using cell phones, the technique used in the Madrid train bombings, were available on the Internet.
The methods, materials and maybe the instructions on when and where to use them, might be just a click away.
Part 1: Man Behind Terror Big's Arrest