Is Trump Right about Cutting off the Internet to Prevent Terror Attacks?

Peter Bergen wrote for CNN about how the Internet impacts terrorist attacks and how it can prevent them: 

As is often the case after a terrorist attack, President Donald Trump quickly took to Twitter after Friday's explosion on the London Underground that injured more than 20 people. Trump tweeted, "Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!"

While the identity of the terrorist or terrorists behind the London explosion is still unknown, Trump is right that many of the "homegrown" terrorists in the West who have carried out attacks in recent years are often failures who find an ideology such as that of ISIS a useful anchor for their grievances. As self-styled ISIS "soldiers," they can become the heroes that they imagine themselves to be, rather than the losers they often actually are.

And Trump is also right that the Internet is now the key recruitment tool for terrorists. The research organization New America has identified 129 militants in the United States who have tried to join terrorist groups in Syria such as ISIS, succeeded in joining such groups or provided support to others joining these groups. Strikingly, in the cases of these 129 militants, there were no instances of in-person recruitment.