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Twitter cannot be held responsible for ISIS' activities on the social network, a federal judge ruled Wednesday in dismissing a lawsuit that accused the company of providing material support to the militant Islamic group.

The lawsuit, which was filed in January by the widow of an American killed in Jordan, accused Twitter of knowingly permitting ISIS to use its network to spread propaganda, attract new recruits and raise funds for terrorist activities. While noting that the ISIS-related killings were "horrific," US District Judge William H. Orrick ruled that the Communications Decency Act immunizes Twitter from liability for the content created by third parties. But he also ruled that plaintiff could file an amended lawsuit.

Neither Twitter nor plaintiff's attorneys immediately respond to requests for comment.

With hundreds of millions of people logging in each day, Islamic State and other radicals have found social media to be a fertile recruiting ground. In response, social networks have increased the size of teams overseeing posts and traffic.