In specific narrow domains like terrorism, companies have adopted blacklists of previously identified material, but in terms of proactively preventing new illegal and harmful content from being posted in the first place, the companies have largely struggled.
The FBI can't "get in a Twitter war with the president," McCabe said Wednesday. "It's hard to publicly fight back against conspiracy theories."
Tech giants Facebook, Twitter and Google are taking steps to police terrorists and hate groups on their sites but more work needs to be done.
President Trump said in a television interview Friday that he was ready to apologize for retweeting anti-Muslim videos from a far-right British group.
Twitter's rules state users "may not make threats of violence or promote violence, including threatening or promoting terrorism." This includes direct as well as indirect threats.
Since February, "the world has witnessed a further wave of deadly, abhorrent terror attacks across the globe," Twitter said in a blog post.