Homeland Security Secretary Jeh Johnson said Tuesday there are “certain legal limits” that constrain federal officials from scrutinizing the social media histories of foreigners trying to enter the United States — a new debate that has flared in the aftermath of the San Bernardino, California terrorist attack.

His comments, in an interview with POLITICO, mark the first time the Homeland Security chief weighed in on the merits of reviewing social media in immigration cases. According to recent news reports, Tashfeen Malik, the female shooter in the California massacre, had posted extremist views yet still obtained a visa to the United States.

“You have to keep in mind — and this is again, not a comment on any particular case — that social media, Facebook, and the like can involve public statements, public postings, it can involve friending, and it can involve private communications,” Johnson said from his office at DHS headquarters in northwest Washington.

“We are dealing with private communications and things for which there is an expectation of privacy, and you’re dealing with U.S. persons,” Johnson continued. “There are certain legal limits to what we can do.”