SAN DIEGO -- Slammer was the fastest computer worm ever, researchers say, spreading to more than 67,000 computers around the world in 10 minutes on Jan. 25, closing bank machines, delaying flights and slowing Internet traffic.

The worm, a string of computer code that took advantage of a flaw in Microsoft Corp.'s server software, doubled in size every 8.5 seconds during the first minute, according to research published by the Cooperative Association for Internet Data Analysis. In contrast, the Code Red worm took 37 minutes -- or more than 250 times as long -- to double, when it appeared in 2001.

Worms are similar to computer viruses in that both types of malicious code make copies of themselves. Worms propagate by attacking a system, while a virus spreads through the exchange of files. Once Slammer infected a computer, it scanned the Internet and sent copies of itself to other vulnerable servers.

Within 10 minutes, Slammer was able to scan 3.6 billion of the world's roughly 4 billion Internet addresses to seek out potential targets, Moore said.

The worm, also called Sapphire, looked for vulnerable computers at a pace of 55 million a second within three minutes of its appearance, slowing only because so much of the worldwide computer network lacked the capacity to allow it to spread as quickly as it could.

Half of all Internet signals weren't reaching their destination at the height of the attack.