Denial of service attacks against Web sites are not uncommon, and are usually caused when sites are deluged with Internet traffic so as to effectively take them off-line. Mounting such an attack can be relatively easy using widely available hacking programs, and they can be made far more serious if hackers infect and use thousands of computers tied together into "botnets."

For instance, last summer, in the weeks leading up to the war between Russia and Georgia, Georgian government and corporate Web sites began to see "denial of service" attacks. The Kremlin denied involvement, but a group of independent Western computer experts traced domain names and Web site registration data to conclude that the Russian security and military intelligence agencies were involved.

Documenting cyber attacks against government sites is difficult, and depends heavily on how agencies characterize an incident and how successful or damaging it is.

Government officials routinely say their computers are probed millions of times a day, with many of those being scans that don't trigger any problems. In a June report, the congressional Government Accountability Office said federal agencies reported more than 16,000 threats or incidents last year, roughly three times the amount in 2007. Most of those involved unauthorized access to the system, violations of computer use policies or investigations into potentially harmful incidents.

The Homeland Security Department, meanwhile, says there were 5,499 known breaches of US government computers in 2008, up from 3,928 the previous year, and just 2,172 in 2006.