The decision follows an attack on Google's servers in mid-December that targeted the Gmail accounts of Chinese human rights activists, Google said in a .

The search giant said it was not the only company targeted.

“As part of our investigation we have discovered that at least twenty other large companies from a wide range of businesses -- including the Internet, finance, technology, media and chemical sectors -- have been similarly targeted,” wrote David Drummond, chief legal officer for Google.

The company said that it is in the process of notifying those companies and is working with U.S. authorities.

(More to follow.)