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Yesterday, al-Qaida's affiliate in Yemen claimed responsibility for the Charlie Hebdo attack. And then last night, the Pentagon announced that five detainees held at the prison camp in Guantanamo Bay are being transferred to other countries. The men who were freed all come from Yemen. Senate Republicans are moving to block any more transfers from Guantanamo. NPR's David Welna reports on how new terrorism attacks are complicating efforts to close the prison.

DAVID WELNA, BYLINE: The five Yemenis being transferred from Guantanamo to Oman in Estonia bring to 33 the number of detainees released over the past year. That's far more than in previous years. All had been cleared for release early in President Obama's administration. Kenneth Roth is the head of Human Rights Watch. He says Obama's clearly trying to fulfill his promise of closing Guantanamo.

KENNETH ROTH: If you compare the population of Guantanamo today with when Obama took office, he's basically cut it in half. He's gone from 241 detainees to 122.

WELNA: Late last month, the president explained on CNN why he's so determined to shut down the prison camp.

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PRESIDENT BARACK OBAMA: It is something that continues to inspire jihadists and extremists around the world, the fact that these folks are being held. It is contrary to our values. And it is wildly expensive. We're spending millions for each individual there.

WELNA: About $3 million a year per prisoner according to the Pentagon. Roth of Human Rights Watch says he thinks cost is a key part of Obama's strategy.

ROTH: I think his plan is to reduce the number sufficiently so that the cost per detainee becomes prohibitive for those who remain, and he's able to overcome some of the congressional resistance to enabling him to transfer the last few detainees.

WELNA: Earlier this week, that congressional resistance was on full display at a news conference at the Capitol. It featured several leading Republican senators on national defense issues, including New Hampshire's Kelly Ayotte.

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SENATOR KELLY AYOTTE: It's one thing to make a campaign promise. But if you look at the security situation that we're facing around the world right now, now is not the time to be emptying Guantanamo with no plan for how and where these individuals are going to go, no assurances of security of those who have been released.

WELNA: Ayotte and her fellow Republicans, including the new chairman of the armed services and intelligence committees, are proposing a two-year moratorium on the transfer of most of the detainees in Guantanamo and barring any transfers to Yemen. South Carolina's Lindsey Graham says the events in Paris show now is not the time to be freeing those held at Guantanamo.

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REPRESENTATIVE LINDSEY GRAHAM: The only reason most of these people have not planned another 9/11 at Guantanamo Bay is because they've been in jail. You let them out of jail, they're going to be on the ground floor planning another 9/11.

WELNA: And Arizona's John McCain, who now leads the Armed Services panel, has done something of an about-face on Guantanamo. The former Vietnam prisoner of war argued for years that Guantanamo was a blight on the U.S. image around the world. He now says many being held there are too dangerous to be released.

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SENATOR JOHN MCCAIN: We're going to continue to release batches of prisoners, according to this administration, with no plan and the extreme likelihood that approximately one out of every three of them will reenter the fight. We owe the men and women who are serving in the military better protection than that.

WELNA: At the White House today, Press Secretary Josh Earnest said only about 6 percent of the detainees released during the Obama administration had returned to the battlefield. Holding prisoners at Guantanamo, he said, only helps jihadists recruit more followers.

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JOSH EARNEST: This is a very difficult policy problem. And it's only been made more difficult by members of the United States Senate who have thrown up obstacles to the president's effort to try to close the prison.

WELNA: An effort that the White House has vowed will continue. David Welna, NPR News, Washington.

CORNISH: And we'll have more analysis on the global security risk related to Yemen tomorrow morning. That's where Morning Edition's David Greene will talk with an expert who says extremists are taking advantage of that country's lack of security infrastructure. Transcript provided by NPR, Copyright NPR.