Republicans and conservative media commentators began attacking the use of the civilian system for terrorism cases in late 2009 and early 2010. They pushed back against a short-lived Obama administration plan to bring five Guantánamo detainees accused of aiding the Sept. 11 attacks to New York for a civilian trial, and deplored the F.B.I.’s handling with criminal justice system procedures of a Nigerian who tried to blow up a Detroit-bound plane with a bomb in his underwear.
The critics maintained that the government would lose information if interrogators read suspects Miranda warnings that they had a right to remain silent and be represented by defense lawyers. They also argued that civilian trials presented a greater risk of spilling classified information and that terrorism trials could turn courthouses into targets.
The talking point that the military should exclusively handle terrorism cases persisted for years among Republicans, even as in scores of such cases, F.B.I. interrogators proved adept at persuading suspects to talk and federal prosecutors swiftly won harsh sentences without security problems. That includes such high-profile cases as the 2010 Times Square car bombing attempt and the 2013 Boston Marathon bombing cases.
At the same time, the Guantánamo tribunals — which had started slowly, after the Supreme Court struck down an earlier version in the Bush years — have repeatedly plunged into chaos. Contested cases, including the attempted prosecution of the five Sept. 11 defendants, bogged down in years of hearings without getting to trial.
“Time and time again, federal courts have proven to be more efficient and more effective than military tribunals,” said Nicholas J. Lewin, a former counterterrorism federal prosecutor. “On punishment, federal courts win. On length of time between capture and conviction, federal courts win. On certainty of upholding conviction on appeal, federal courts win.”
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Amid those real-world complexities, the Trump administration has confounded expectations that it would swiftly bring new detainees to Guantánamo. It has brought to civilian court several foreign terrorism suspects, including an Algerian-Irish Qaeda suspect facing trial in Philadelphia and a Turkish Islamic State suspect facing trial in New York.
Attorney General Jeff Sessions, who as a senator routinely attacked the Obama administration as foolhardy for not sending newly captured terrorists to Guantánamo, is now extolling the successes of civilian courts.
“Since 9/11, the Department of Justice has made fighting terrorism its top priority,” Mr. Sessions said in a speech on Thursday in New York. “Our goal is not just to catch terrorists, but to prevent them from striking us. And in this fight against terror, we have gotten results.”
Mr. Sessions did make a nod to Guantánamo, citing it as another lawful tool that the Trump administration remains willing to use. But that fleeting and vague reference contrasted with the detailed praise he heaped on civilian terrorism prosecutions.
Mr. Sessions lauded the conviction last month of Ahmad Khan Rahimi in the September 2016 bombing in the Chelsea neighborhood of Manhattan; he faces a mandatory life sentence. Mr. Sessions pointed to charges unsealed last month against three men accused of plotting Islamic State-inspired attacks in New York.
And he cited the case of Mustafa al-Imam, the second suspect captured in the 2012 attacks in Benghazi, Libya, who was seized by American commandos in a raid there last week and made his initial appearance on Friday in a federal courthouse in Washington.
Several signs have emerged this year that Mr. Sessions has begun to view the practical merits of Guantánamo-style policies more skeptically in his new job.
In March, for example, he said in a radio interview that it was time to think through “to what extent we’re going to use military commissions” because they were not working “in an effective way,” although he expressed hope of fixing them. And in July, when Mr. Sessions visited Guantánamo, he privately expressed dismay at the expense of the prison operation, according to multiple American officials familiar with his comments. Taxpayers spend millions of dollars a year on each detainee, roughly 100 times the cost of housing inmates in high-security civilian prison.
Another senior presidential adviser, Thomas P. Bossert, the homeland security adviser, suggested at a meeting this year a compromise to make the prison somewhat more useful for new captives: take suspects to Guantánamo for a period of interrogation, then bring them to the United States for civilian trial. But he was told, according to a current and a former American official, that a law passed by Congress to block Mr. Obama’s plan to close the prison barred the transfer of detainees to domestic soil once they have been held at Guantánamo.
A spokesman for Mr. Bossert did not comment, and a spokesperson for Mr. Sessions did not respond to an inquiry. But Matthew Miller, the Justice Department spokesman under former Attorney General Eric H. Holder Jr., expressed vindication.
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“Their opposition to trying terrorists in federal court was always based on nothing but politics, and it was always bound to collapse the minute a Republican became president and had to face reality,” Mr. Miller said.
He predicted that under the next Democratic president, some Republicans would revive the issue “to score political points.”
Among the Republicans who have been strikingly quiet on the topic this year is the Republican Senate leader, Mitch McConnell of Kentucky. He once led the charge, telling fellow Republicans in 2010 that they could reap political gains by campaigning on the issue. And he repeatedly followed that playbook himself.
In 2011, for example, when two Iraqi refugees in Kentucky were charged with committing terrorism offenses abroad, Mr. McConnell denounced the move. “The perfect place for these terrorists is at Guantánamo,” he told Fox News.
And in 2014, when American commandos captured Abu Ahmed Khattala, a Libyan suspect in the Benghazi attacks, Mr. McConnell maintained that interrogating him under law enforcement procedures would be a mistake because the United States needed to learn who else was involved in the attack.
But, following its procedures, the F.B.I. first questioned Mr. Khattala for intelligence purposes, and then, after advising him of his rights, started over for information that could be used in court. At his continuing trial in Washington last week, an agent who interrogated Mr. Khattala testified that the F.B.I. had succeeded in getting him to identify Mr. Imam, the just-captured second Benghazi suspect.
“Mr. Imam, we didn’t know who he was until Abu Khattala told us,” the F.B.I. agent said.
In an email, a spokesman, Don Stewart, said that Mr. McConnell’s position on sending foreign terrorism suspects to Guantánamo had not changed. But Mr. Stewart also said that Mr. McConnell had never had a blanket policy. He pointed out that the truck attack suspect, Mr. Saipov, was probably ineligible for wartime detention because no evidence had emerged that he was part of an organization with which the United States is at war.
That did not stop Senator Lindsey Graham, Republican of South Carolina, from saying that Mr. Saipov, who waived his Miranda rights and spoke freely to F.B.I. agents, could and should be placed in military custody based on his support for the Islamic State.
Mr. Graham, a lonely voice in loudly criticizing Mr. Trump’s decision to stick with the civilian system, insisted in an interview that Mr. Saipov should have been interrogated in military custody. Mr. Graham scoffed at the notion that the F.B.I. would learn all he knew — especially once he had a lawyer.
But he also acknowledged that some of his colleagues who once shouted in unison with him were now quiet.
“If Obama had done this,” he said, “there would have been a lot more Republicans out there.”