Defense Secretary Ash Carter said Tuesday that he supports the president's plan to close Guantanamo Bay by the end of this administration, but that it may not be realistic given the large population of prisoners who can't be released.

"If would be a nice thing to do and an important thing to do if we can do it, but we've got to be realistic about the people who are in Guantanamo Bay. They're there for a reason," Carter told service members via video based on a question from a sailor serving at Guantanamo Bay.

Carter said about half of the Guantanamo Bay population can be transferred to another country with security restrictions to ensure they don't return to the battlefield. The other half, however, needs to be detained indefinitely because they pose too great a risk.

"If they're not locked up in Guantanamo Bay, they need to be locked up somewhere. So we are looking at places in the United States, prisons and other places, to which these people can be moved," Carter said.

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