The idea of checking foreigners’ social media posts, which remains limited to a preliminary discussion, draws on a supposed history of terror attacks where the attacker had previously expressed extremist views on platforms like Facebook and Twitter.

Some have pointed to the San Bernardino terror attack as evidence that such a policy might be useful: An FBI document produced shortly after the shooting said that the woman who helped carry it out pledged allegiance to ISIS while the attack was ongoing. FBI director James Comey later confirmed, however, that the attackers — Syed Rizwan Farook, 28, and Tashfeen Malik, 29 — expressed support for “jihad and martyrdom” in private communications but never did so publicly on social media.

It is unclear whether the social media mandate would be constitutional. Legal challenges have already been presented to Trump’s “extreme vetting” order, and large protests erupted at airports across the country on Saturday as news emerged that people from the banned countries, who had valid visas and green cards, were being detained — and, in some cases, deported — by customs officials and border patrol agents.