The secretary of the Department of Homeland Security said Tuesday that “extreme vetting” for travelers to the United States could include recording the websites they visit and the contacts in their cell phones.

Asked by a reporter what the “extreme vetting” in President Donald Trump’s immigration order might entail, DHS Secretary John Kelly said visa applicants would have to prove their identity, possibly by surrendering personal digital information.

“It might be certainly an accounting of what websites they visit. It might be telephone contact information so that we can see who they’re talking to, but, again, all of this is under development,” he said. “Those are the kind of things we’re looking at. Social media. We have to be convinced that people that come here, there’s a reasonable expectation that we don’t know who they are and what they’re coming here for and what their backgrounds are.”

Kelly specified at the top of his answer that the seven Muslim-majority countries singled out by Trump’s executive order “don’t have the kind of law enforcement, records keeping, that kind of thing that can convince us that one of their citizens is, indeed, who that citizen says they are and what their background might be.”

That, he said, would be the motivation for recording personal digital information.

“Right now there are a number of countries on the planet that don’t have that kind of records keeping, police work, that kind of thing, and the seven in question right now for the most part fall into that category,” he said. “So we are developing what additional vetting, extreme vetting might look like, and we will certainly work with countries in this.”