The website of a network of domestic and overseas China rights activists, Chinese Human Rights Defenders—or CHRD— was severely hacked on Saturday.

CHRD says the cyber attack lasted 14 hours—overloading its server and paralyzing its website. They released a statement suggesting a large number of computers and a great deal of planning probably went into the attack.

An IT expert associated with CHRD said, “I don’t think ordinary Chinese hackers have this kind of capability. The Chinese government is the most likely suspect for these attacks.”

CHRD says they have been the target of cyber attacks frequently, especially during ‘sensitive’ periods in China.

The Chinese regime has denied any involvement in cyber attacks.

Four other organizations: a human rights group, Civil Rights and Livelihood Watch; a writers’ association, Independent Chinese Pen; and two news websites, New Century News and Canyu also suffered cyber attacks on Saturday.

The attacks happened just after U.S.-based search engine Google threatened to pull out of China earlier this month. It was over a spate of cyber attacks on the email accounts of Chinese rights activists and ongoing Internet censorship issues.