ALEXANDRIA, Va. — A federal jury on Thursday convicted a British militant accused of being a member of the brutal Islamic State cell known as the Beatles in the abduction, abuse and deaths of four Americans, a major victory for U.S. prosecutors and the families of victims who sought to bring him to justice.

The jury took less than a day to convict El Shafee Elsheikh, 33, on four counts of hostage-taking and four conspiracy counts related to the deaths of three American men and a young woman who were captured during the Islamic State’s rampage through Syria in 2012 and 2013.

Mr. Elsheikh is the most prominent member of the Islamic State to be brought to trial in the United States. He was captured in Syria by a Kurdish-backed militia in 2018, along with Alexanda Kotey, as they tried to flee to Turkey. Mr. Kotey, 38, who was part of the Beatles, said last fall that he had played a critical role in the kidnapping, detention and hostage negotiations of American prisoners and pleaded guilty to multiple charges.

The verdict capped a two-week trial that featured the testimony of 35 witnesses, including 12 former captives who detailed relentless beatings, sexual abuse, waterboarding and murder perpetrated by a cell of four radicalized young Britons, nicknamed the Beatles for their accents and sarcastic banter.

Prosecutors have argued that the polite, bespectacled defendant was a central figure in the Islamic State hostage conspiracy, responsible for drafting ransom emails and mistreating prisoners. Among those captives, they say, were Kayla Mueller and three American men — James Foley, Steven J. Sotloff and Peter Kassig — who were later beheaded by one of Mr. Elsheikh’s close associates.

Mr. Elsheikh did not deny fighting for the Islamic State, but in rebutting the charges, his defense team argued that he was not a member of the Beatles and his purported involvement in the kidnappings was a case of mistaken identity.

In his closing remarks on Wednesday, the first assistant U.S. attorney, Raj Parekh, asked jurors to pay particular attention to the suffering endured by Ms. Mueller, 24. She was not only physically abused like the other American captives, but treated as a slave in the months leading up to her death, under mysterious circumstances, in early 2015.

Mr. Elsheikh has not been directly implicated in the killings, but his participation in — and knowledge about — numerous kidnapping, ransom and murder plots is enough to secure a conviction under the law, prosecutors have argued.

The British extremists repeatedly beat the hostages they kept imprisoned in Raqqa, Syria, which the Islamic State claimed as its capital at the time, according to prosecutors. They subjected their hostages to abuses including waterboarding, mock executions, painful stress positions, food deprivation, chokeholds that caused blackouts, electric shocks and beatings that lasted 20 minutes or longer. They also forced the prisoners to fight one another and to witness killings, court papers said.

During the trial, the government introduced testimony from freed hostages who detailed the sadism of the cell members. But the hostages were often blindfolded, and their captors were careful to always wear masks, making definitive physical identification difficult.

The prosecution team relied heavily on Mr. Elsheikh’s public comments about his actions. He gave at least seven news interviews after being captured by Kurdish forces and turned over to the U.S. military in 2018, disclosing knowledge of key operational details and his own role in seeking to extract millions in ransom payments for Western hostages.

Mr. Elsheikh’s appearance in an American courtroom is a result of intense political and legal wrangling. In August 2020, William P. Barr, the attorney general at the time, agreed to waive the death penalty against Mr. Elsheikh and Mr. Kotey in exchange for cooperation from British prosecutors — seen as a key element in obtaining a conviction.

As part of the plea deal, if Mr. Kotey fulfills his cooperation requirements, he could be sent to Britain after 15 years to complete the remainder of a mandatory life sentence.