Chinese censors are investigating 52 websites owned by firms including top search engine Baidu Inc and online video business Youku Tudou Inc over the distribution of violent and pornographic content, the official Xinhua news agency said on Thursday. The investigation comes as Chinese authorities tighten their control over the Internet. In April, Chinas national anti-pornography office stripped Sina Corp of some online publication licenses as part of the crackdown.. The latest websites under investigation, which include a web portal owned by Chinese Internet firm NetEase, are suspected of distributing shows such as cartoon series Grimms Fairy Tales that contain violent and obscene content, Xinhua said, without citing sources.