​The United States government official who vowed to shut down the detention center at Guantanamo Bay announced this week that he’ll be resigning from that role at the end of the year.

Clifford Sloan – the US State Department’s envoy to Gitmo, tasked with assisting in the facility's closure – said on Monday that he’d be leaving his job on December 31 following an 18-month term. Reuters called the news a “surprise announcement,” but Sloan told The New York Times that he always intended on staying on the job for just a year-and-a-half. However, the Chicago Tribune cited an unnamed senior US official as saying that the Department of Defense may have played a role, as the Pentagon "certainly hasn't been as helpful as they could have been" in closing the detention center.

The administration of US President Barack Obama appointed Sloan – formerly a Washington-based lawyer – to help the State Department shut down the detention center back in mid-2013, when the military-run jail held 166 detainees. The White House’s recent decision to repatriate four Afghan prisoners long-contained at Gitmo dropped the inmate count down to 132, The Wall Street Journal reported.

“At this point, we’re in a position to see a lot of progress [in regards to the prison's future],” Sloan, 57, told the Times. “I’m strongly in favor of moving forward as promptly as we can on the president’s commitment to close the facility.”

But while Sloan saw more than 30 detainees taken out of the detention center on his watch during the last 18 months, his impending resignation will all but certainly occur without him having achieved a total shutdown, as he long hoped for.

“We are going to close the Guantanamo detention facility. I have no doubt about that. And President Obama is very strongly committed to that,” Sloan said to PBS back in January.

Efforts from the Obama administration have been largely unsuccessful, however, at least with respect to the president’s pre-election campaign promise of closing down the facility for good.

On Friday, Obama told CNN host Candy Crowley that he’s “going to be doing everything I can to close” Guantanamo.

"It is something that continues to inspire jihadists and extremists around the world, the fact that these folks are being held," the president said.

Nevertheless, Secretary of State John Kerry hailed Sloan this week upon news of his resignation, and said the envoy was responsible for overseeing and negotiating the transfer of 34 detainees during the last 1.5 years.

“I’d like to have about a hundred Cliff Sloans. He’s the real deal,” Kerry said in a statement. “Believe me, there were a lot of doubters. This wasn’t the most coveted job in Washington. A lot of people thought Cliff must not have known what he was taking on when he signed up here.”

Sloan has been “the model of someone very successful on the outside who comes in to the State Department and builds relationships instead of burning bridges, gets people on board with a tough assignment, masters the inter-agency process and just keeps his head down and proves the doubters dead wrong,” Kerry said.

Explaining his time as State Dept. special envoy, Sloan told Foreign Policy on Monday that he was “honored to have been able to play a part in making progress” when it came to fulfilling the president’s promise.

“I’m very pleased with the progress we’ve made,” he told The Wall Street Journal, “and we’re poised to make very significant progress in the weeks and months ahead.”

According to Kerry, more Gitmo inmates will soon be released from the prison. The Obama administration has taken heat for its efforts so far, though, particularly by Republican Party opponents who say that maintaining the detention center is necessary for national security.

“What the Obama administration is doing is dangerous and, frankly, reckless,” Buck McKeon, the retiring chairman of the House Armed Services Committee, told the Guardian last month with regards to the repatriating of former Gitmo detainees authorized recently by the White House.

“It’s ridiculous, because if you think about the people who are being held at Guantanamo, they are very dangerous terrorists,” Sen. Kelly Ayotte (R- New Hampshire) said on Fox News.

Having once held 779 detainees, today only 132 prisoners remain at Gitmo. Last week, the US released four Afghan detainees who had been held for more than a decade without charge.