The other is to block the electronic path to objectionable material with filtering software.

ECONOMIST: China and the Internet

But because filtering programs use keywords to block material, they can inadvertently restrict content you want your children to access.

CNN: Tools: Protecting your kids online

Rigorous filtering targets political, human rights, religious material and content deemed obscene.

BBC: Bahrain profile

"There's no guarantee that they are going to be able to get the offensive material out of the system quickly, " said Geoff Haggart, European head of filtering firm WebSense.

BBC: The problems of finding filth

The possible presence of additional Web-filtering machines, despite the earlier investigation, underscores problems with the enforcement of embargoes: Once a company sells material to an exporter legally, it isn't obliged to track where the goods end up.

WSJ: U.S. Firm's Gear Seen Aiding Syria