NEW : Users in China are already getting alerts , one ex - journalist said . Google wo n't say how it knows we are behind the hacking . Security chief says it does n't want to give away anything that could help the investigation . The Google Watch warns users to change passwords and upgrade software .
NEW : Users in China are already getting alerts , one ex-journalist says . Google wo n't say how it knows governments are behind the hacking . Security chief says it does n't want to give away anything that will help the hackers . The Internet giant warns users to strengthen passwords and upgrade software .