The fate of Mr. Darbi has been closely watched for several reasons. If the United States lived up to the Obama-era deal, it would mean that President Trump would preside over a reduction of the Guantánamo detainee population, which would drop from 41 to 40, despite his campaign vow to fill it back up.
But if the government reneged on the deal, legal experts have warned that it would strongly discourage other detainees from cooperating with military commissions prosecutors by pleading guilty and potentially serving as witnesses — a key tool that civilian court prosecutors routinely use to win convictions.
Last year, the State Department, under Secretary of State Rex W. Tillerson, dismantled the Office of the Special Envoy for Guantánamo Closure, which had been created by the Obama administration to make diplomatic arrangements for detainee transfers and to handle any subsequent diplomatic issues regarding former detainees. A remaining official was reassigned to help process Freedom of Information Act requests.
In November, shortly after Mr. Darbi’s sentencing, the State Department, in response to a question by The New York Times about whether anyone was actively working on transfer arrangements, said only that it was prepared to support the Pentagon in implementing the plea agreement. At the time, it was unclear what the Pentagon was doing.
On Tuesday, Lee Wolosky, who as the last of several Obama administration special envoys for Guantánamo closure arranged for the transfer of more than a dozen detainees to Saudi Arabia between late 2015 and early 2017, criticized the Trump administration’s approach to getting that task done.
“It takes weeks or months to lay the groundwork for a normal transfer (which this one is not), from arranging logistics, to negotiating security arrangements, to obtaining the necessary political approvals,” he wrote in a text message. “The D.O.D. historically has not interfaced with the right parts of the Saudi government to get all that done.”
On Tuesday, Commander Higgins acknowledged that the Defense Department is taking the lead in talks with the Saudi government about transferring Mr. Darbi. But she declined to address how long those negotiations have been taking place or what assurances the Pentagon is seeking but has not received.