F.B.I. agents are also hoping to bring back and try a pair of American terrorism suspects being held in Turkey, according to former and current United States officials who spoke on the condition of anonymity to discuss internal deliberations. Under current law, the Guantánamo military tribunals may not be used to prosecute American citizens, although Mr. Trump said during the campaign that he was “fine” with expanding their use to encompass citizens.

President George W. Bush opened the prison in January 2002 as a place to bring captives from the fighting in Afghanistan in the months after the Sept. 11 terrorist attacks, and nearly 800 prisoners were eventually brought there. After the prison’s image became toxic around the world and the Supreme Court ruled that federal civilian courts had jurisdiction to review habeas corpus lawsuits by detainees there, the Bush administration began trying to close it, reducing its population to 242 men by the time Mr. Bush left office.

President Barack Obama continued that policy, ordering the prison closed within a year as one of his first acts upon taking office in 2009. But, as political winds shifted, that proved easier said than done, and he failed to complete the task. Still, by the time he left office in January, the detainee population had been reduced to 41 men.

By contrast, Mr. Trump vowed during the campaign to keep the prison open and fill it again, an aspiration that was echoed in the early draft executive orders. Some legal specialists have warned that it is not clear that the government’s wartime authority to fight the perpetrators of the Sept. 11 attacks — that is, Al Qaeda — covers the Islamic State, and so bringing in a detainee tied to that group could create legal risks.

In April, American military officials had hoped to capture a top Islamic State terrorist in Syria. But the man, Abdurakhmon Uzbeki, was killed in a firefight with American commandos. Officials said that federal prosecutors had been preparing civilian criminal charges against him for possible prosecution in the United States.

This year, Mr. Sessions expressed frustration at the pace of military commissions. Two major death-penalty cases — one over the 2000 bombing of the naval destroyer Cole, and one against Khalid Shaikh Mohammed and four other detainees accused of playing a role in the Sept. 11 attacks — have languished in pretrial hearings for years. Mr. Sessions said then that it was time to figure out how use the system in an effective way.

Last month, military commission prosecutors charged a longtime detainee best known as Hambali with two deadly bombings in Indonesia in 2002 and 2003, setting in motion what could be the first new tribunal case of the Trump era. The chief defense counsel said that prosecutors had informed him that they would not seek the death penalty if the so-called convening authority, which oversees the system, permitted the case to proceed.