Obama promised during the presidential campaign to shut Guantanamo, endearing him to constitutional law experts, civil libertarians and other critics who called the Bush administration detentions a violation of international law.

But he acknowledged in an interview Sunday that the process of closing the prison would be harder and longer than initially thought.

"That's a challenge," Obama said on ABC's "This Week." "I think it's going to take some time and our legal teams are working in consultation with our national security apparatus as we speak to help design exactly what we need to do.

"But I don't want to be ambiguous about this," he said. "We are going to close Guantanamo and we are going to make sure that the procedures we set up are ones that abide by our constitution."

President George W. Bush established military tribunals to prosecute detainees at Guantanamo. He also supports closing the prison, but strongly opposes bringing prisoners to the United States.

Lawmakers have moved to block transfer of the detainees to at least two potential and frequently discussed military facilities: an Army prison at Fr. Leavenworth, Kan., and a Navy brig in Charleston, S.C. A Marine Corps prison at Camp Pendleton in Southern California also is under consideration, a Pentagon official said.

Sen. Sam Brownback, R-Kan., said Monday that "it's hard to show why terror suspects should be housed in Kansas."

"If the holding facility at Guantanamo Bay is closed, a new facility should be built, designed specifically to handle detainees," Brownback said in a statement.

A Pentagon team also has been looking at how to shut Guantanamo and move its detainees but spokesman Bryan Whitman did not immediately know Monday whether it was completed.

The executive order marks only a first step at what is likely to be a long legal process. Still, American Civil Liberties Union legislative director Caroline Fredrickson called "extremely meaningful" even if the Guantanamo prison can't be closed immediately.

"It's clear that there is a process of time that will be necessary to close it properly, to make sure that human rights and respected and security is protected," Fredrickson said. "But the fact that it's set in motion is extremely good news."