The Pentagon is preparing to build temporary camps for immigrants at two military bases, Defense Secretary Jim Mattis said Sunday.

He did not name the two bases, but said the details are being worked out, including how much capacity is needed. The Pentagon had initially talked about four potential bases, but Mattis indicated the number is now two.

The Pentagon last week said it would make space available on military bases for as many as 20,000 unaccompanied migrant children detained after illegally crossing the U.S.-Mexico border. It wasn’t clear Sunday if the housing would be limited strictly to children or if it would also involve families.

The victims were Vietnamese tourists who had arrived in Vegas the day...