Some recent international cyber attacks

A look at some known international cyber attacks in recent years:

-In April, a former U.S. government official said that spies had hacked into the U.S. electric grid and left behind computer programs that would let them disrupt service, though it was not clear when the breach occurred. The official said the intrusion was "almost without a doubt" done by state sponsors.

-In March, a Canadian research group concluded that hackers likely based in China stole sensitive information from thousands of hard drives worldwide and hacked into the computer system of the Dalai Lama, the spiritual leader of Tibet. China denied any involvement.

- In March, U.S. Sen. Bill Nelson, D-Florida, said his office computers were hacked three times by "cyber-invaders thought to be inside China." Nelson, a member of the Senate's Intelligence, Armed Services and Finance committees, described one of the incidents as serious, but said he did not believe any sensitive information was stolen.

- In 2008, Georgian government and corporate Web sites began to see "denial of service" attacks just ahead of the outbreak of war with Russia. 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 top security and military intelligence agencies were involved.

- In 2007, alleged Russian hackers crippled government and corporate computer networks in Estonia for nearly three weeks following deadly riots that were sparked by the relocation of a Soviet war memorial.