Epitaphs for the Mubarak government all note that the mobilizing power of the Internet was one of the Egyptian opposition’s most potent weapons. But quickly lost in the swirl of revolution was the government’s ferocious counterattack, a dark achievement that many had thought impossible in the age of global connectedness. In a span of minutes just after midnight on Jan. 28, a technologically advanced, densely wired country with more than 20 million people online was essentially severed from the global Internet.

The blackout was lifted after just five days, and it did not save President Hosni Mubarak. But it has mesmerized the worldwide technical community and raised concerns that with unrest coursing through the Middle East, other autocratic governments  many of them already known to interfere with and filter specific Web sites and e-mails  may also possess what is essentially a kill switch for the Internet.

Because the Internet’s legendary robustness and ability to route around blockages are part of its basic design, even the world’s most renowned network and telecommunications engineers have been perplexed that the Mubarak government succeeded in pulling the maneuver off.

But now, as Egyptian engineers begin to assess fragmentary evidence and their own knowledge of the Egyptian Internet’s construction, they are beginning to understand what, in effect, hit them. Interviews with many of those engineers, as well as an examination of data collected around the world during the blackout, indicate that the government exploited a devastating combination of vulnerabilities in the national infrastructure.