GUANTÁNAMO BAY, Cuba — On a Monday morning four and a half years ago, Ramzi bin al-Shibh, one of the men accused of conspiring in the Sept. 11 attacks, made a stunning announcement for all in the military courtroom to hear: He knew the new Arabic translator sitting beside him from the secret C.I.A. prison network where the United States tortured its detainees.
What followed said a lot about the enduring secrecy around the torture program and the black-site prisons, the disputes over what evidence is admissible in the war court established to try terrorism suspects and the grindingly slow effort to get justice in the 2001 attacks.
By blurting out the man's name in open court, Mr. bin al-Shibh had undercut the government's efforts to keep under wraps the identities of most people who worked at the black sites. The interpreter's name, initially included in the court transcript for that day, was later redacted. Defense lawyers were instructed — in secret — that what happened was a security breach and that they were forbidden, by a national security directive, to publicly acknowledge what had happened in open court.
Never mind that the interpreter's name was called out publicly at the Guantánamo air terminal at the end of that week as lawyers and journalists checked in for their flight back to the United States.
The revelation derailed a week of pretrial hearings and was the start of one of the strangest and most complex chapters in the Sept. 11 case's pretrial hearings.
The interpreter has not been seen publicly at Guantánamo since then. And more than four years later, defense lawyers and prosecutors say they did not know about the man's past when he was assigned to be an interpreter for the defendants at the trial.
But the moment raised still unanswered questions of how someone who had worked for the C.I.A. found his way to a defense team that had already protested government efforts to gather intelligence about its activities, from the F.B.I. turning a defense-team member into a confidential informant to the discovery of listening devices hidden in supposedly confidential attorney-client meeting rooms.
There was some initial skepticism about the claim by Mr. bin al-Shibh, a Yemeni captive who is accused of organizing the Hamburg, Germany, cell of 9/11 hijackers. The C.I.A. had deprived him of sleep and subjected him to other memory-damaging abuse during the four years the United States held him incommunicado starting in 2002. Once at Guantánamo, the military drugged him.
It seemed possible that he imagined he knew the man through a haze of tortured memories, even though some of his fellow defendants told the judge they recognized the interpreter too after Mr. bin al-Shibh said to the court: "The problem is I cannot trust him because he was working at the black site with the C.I.A. and we knew him from there."
But Mr. bin al-Shibh was right. During that week in February 2015, people with knowledge of the case confirmed that the linguist had worked for the C.I.A. — and that he lied about it in interviews with defense lawyers when he was assigned to the Guantánamo trial.
The episode has elements of the absurd. But it offers a graphic illustration of why the effort to try, convict and execute the five men accused of conspiring in the hijackings that killed 2,976 people has been mired in pretrial hearings since 2012.
In the years since Mr. bin al-Shibh recognized the interpreter, defense lawyers have for years sought to question him to help them develop their arguments to avert a death-penalty trial for their clients.
They say information from the interpreter could bolster a defense theory that the Bush administration thwarted justice by torturing the suspects in the black sites rather than taking them straight to the United States for trial. And they want to know if the interpreter can fill in gaps left by the destruction of evidence by the C.I.A., to argue the defendants have been deprived of due process.
The interpreter's testimony has taken on greater importance, the lawyers say, because he may be a rare eyewitness to how the defendants were held and interrogated in the black sites.
The prosecutors have been so determined to stop the testimony — or at least restrict it to a secret session — that they put the judge on notice last month: If he opted for open-court testimony, even if delivered in a way that masked the interpreter's identity and distorted his voice, the government would invoke a national security privilege and refuse to let him testify at all.
The law that created the military commissions grants that authority to the team of prosecutors, which is led by Brig. Gen. Mark S. Martins of the Army. But a judge can then freeze the proceedings until the government allows him to testify.
In court several weeks ago, one of the prosecutors, Clayton Trivett, acknowledged that the interpreter's identity had been made public. But he said the man and his family would be in danger even if he were to testify anonymously behind a blank screen through a voice scrambler in the courtroom, where no recording is allowed. The interpreter's name has now been officially classified.
The New York Times has chosen not to identity the interpreter by name in this article. Efforts to reach him failed.
Like nearly every other linguist at Guantánamo, English is his second language and he has an accent. But, the prosecutor argued, a foreign intelligence agent who somehow managed to hear the testimony might be able to track him down.
Anne FitzGerald, an investigator with Amnesty International in London, who saw the interpreter episode occur in the courtroom at Guantánamo in 2015, called the tug-of-war over his testimony "some sort of metaphor for the whole process," which has dragged on for years with few signs of coming to a close anytime soon.
"It was so bizarre in the first place that he appeared there," Ms FitzGerald said. "Either it was an enormous oversight on somebody's part, or not. So it's equally bizarre that they've taken four years to figure out what to do about it."
In one reminder of the slow pace of the case since 2015, Ms. FitzGerald's Amnesty International portfolio has changed from researching counterterrorism to gun violence.
The judge at the time the interpreter appeared in court has retired — and been replaced twice.
Now the new trial judge, Col. W. Shane Cohen of the Air Force, is deciding whether the defense lawyers can question the interpreter about his work with the C.I.A., or just how he found his way to a pool of contract linguists defense lawyers can use at Guantánamo.
To avoid prosecution concerns that someone will recognize the interpreter — again — the judge has canceled scheduled secret testimony in favor of a sworn deposition. Under that formula, neither the accused nor the public will be able to watch when the interpreter answers questions under oath, at a top secret facility in the United States.
The secret testimony almost happened at Guantánamo in January, by order of Colonel Cohen's predecessor.
But that judge, Col. Keith A. Parrella of the Marines, was suddenly diagnosed with a detaching retina and cut short a weeklong hearing before the interpreter testified.
That delay gave defense lawyers, the American Civil Liberties Union and news organizations, The New York Times included, time to challenge the closure of the proceeding in a higher court. The United States Court of Military Commission Review found that Colonel Parrella had not justified closing the unclassified portions of the linguist's testimony, and sent the issue back to the trial court to consider again.
Colonel Cohen heard arguments on the issue last month in what became a test run for how to handle the actual evidence. When a defense lawyer, Alka Pradhan, argued for open-court testimony by the interpreter, a court security officer on two occasions triggered a white noise button, preventing journalists and other members of the public from hearing what she had said.
At the Guantánamo court, the audio emerges from the court to a spectators' gallery on a 40-second delay, time enough for a security officer to block spectators from hearing classified information — something that did not happen the day Mr. bin al-Shibh named his interpreter in open court.
But last month, the public heard Ms. Pradhan's words turn into white noise midsentence. After the first instance, once the sound came back on, Mr. Trivett, a prosecutor who specializes in national security questions, apologized for mistakenly having her silenced.
"I believed that was national security information," he said, according to a transcript of the unclassified session. "I was incorrect."
The same transcript blacked out two of Mr. Trivett's open-court remarks, for the first time, on why the interpreter should not testify in open court.