ALEXANDRIA, Va. — A federal judge on Friday sentenced a British terrorist to life in prison for his central role in the kidnap, torture and killing of Western hostages who were held by the Islamic State in Syria.

Alexanda Kotey, 38, was part of a brutal ISIS cell of Britons called the Beatles, a nickname given by their victims because of their accents. Mr. Kotey pleaded guilty last year to multiple charges that guaranteed he would spend the rest of his life behind bars.

Mr. Kotey admitted he was directly involved in the detention and hostage-taking of four Americans in 2012 and 2013, all of whom had traveled to Syria as journalists or to provide humanitarian aid and died in Islamic State custody: James Foley, Kayla Mueller, Steven Sotloff and Peter Kassig. Mr. Foley and Mr. Sotloff were beheaded in gruesome propaganda videos that stunned the world.

The executioner of the group, Mohammed Emwazi, also known as Jihadi John, was killed in an airstrike in Syria in 2015.

During the sentencing on Friday, British and American families read statements describing their shattered lives. Mr. Kotey sat quietly and listened to parents and siblings recount their horror before and after the deaths of their loved ones.

“We are forever broken by the loss of our beloved son,” said Shirley Sotloff, whose son was executed in the Syrian desert in September 2014. “Open your eyes, please, and look at me. You destroyed our lives and we hope for the rest of your lives you will think about what you have done, and to your families as well.”

After the families, along with a pair of Syrian detainees who had been captured with two of the Americans and later freed, concluded their comments, Judge T.S. Ellis III of the Federal District Court praised the dead hostages, saying they had moral courage and compassion.

He said the prosecution of Mr. Kotey was not revenge but the search for truth, describing Mr. Kotey’s conduct as “egregious, violent and inhumane.”

Judge Ellis sentenced Mr. Kotey to eight life sentences, which are to run at the same time and could land him at a high-security prison in Florence, Colo., called the supermax. Mr. Kotey’s lawyers had asked the judge to recommend sending Mr. Kotey to a prison with less severe restrictions, but he declined.

Mr. Kotey declined to comment, instead referring to a lengthy letter he had filed with the court. The letter describes at times feeling sympathy for his captives, but such sentiments, he wrote, were “often brief and momentary.” He called himself a soldier carrying out orders and using harsh tactics that were necessary to fight the United States as part of asymmetrical warfare.

While Mr. Kotey said in the letter that he took responsibility for his actions and that his own capture and detention had led to reflection, he did not apologize for his crimes, writing that he was “slightly reluctant to use such terms like remorse.”

As part of his plea deal, Mr. Kotey agreed to meet with the families of the dead hostages and share information with F.B.I. agents and prosecutors, a process that has already started. If he fulfills his cooperation requirements, Mr. Kotey could be sent to Britain after 15 years to complete the remainder of his life sentence.

Earlier this month, a jury swiftly convicted El Shafee Elsheikh, 33, another Islamic State militant believed to be a member of the Beatles, on four counts of hostage-taking and four counts of conspiracy related to the deaths of the Americans.

At Mr. Elsheikh’s trial, Raj Parekh, the first assistant U.S. attorney, said Mr. Kotey and Mr. Elsheikh were terrorists and brothers in arms who reveled in sadism and were deeply tied to the kidnapping scheme.

“The evidence demonstrates that they grew up together, radicalized together, fought as high-ranking ISIS fighters together, held hostages together, tortured and terrorized hostages together,” Mr. Parekh said. “And after Emwazi was killed, Elsheikh and Kotey were ultimately captured in Syria together.”

Prosecutors recounted how the hostages had been beaten and waterboarded and endured mock executions. One Danish hostage was hit 25 times in his ribs on his 25th birthday, Mr. Parekh said. Some were freed after ransoms were paid.

Mr. Elsheikh, who decided not to strike a deal with prosecutors, is scheduled to be sentenced in August. The judge in the case had ordered him to be in the courtroom on Friday, and he said nothing at the hearing, sitting motionless next to his lawyers and not far from Mr. Kotey.

Mr. Kotey and Mr. Elsheikh were captured in Syria by a Kurdish-backed militia in 2018 as they fled to Turkey.

The prosecutions of Mr. Kotey and Mr. Elsheikh were not always a certainty because the British government declined to provide crucial evidence for a trial if the Justice Department insisted on being able to seek the death penalty in the cases.

Eventually, William P. Barr, the attorney general at the time, decided to rule out the death penalty, paving the way for their prosecutions in federal court in Alexandria, Va.

The outcome in the cases bolstered the argument that civilian courts were more effective than military tribunals in trying terrorists.

Carl Mueller, the father of Kayla Mueller, told the court that he had lost faith in God and his government after his daughter, a humanitarian worker from Arizona in her 20s, was abducted and then raped repeatedly by Abu Bakr al-Baghdadi, the slain leader of the Islamic State. Ms. Mueller was likely killed by the Islamic State in February 2015 after about 18 months of captivity.

Mr. Kotey and Mr. Elsheikh were “cowards,” Mr. Mueller said, adding that watching the trial had “restored my faith in the government.”