WASHINGTON — A court on Wednesday barred the British government from providing U.S. prosecutors with evidence against two Islamic State militants suspected in the beheadings of Western hostages, citing the prospect the men could face the death penalty if tried and convicted in America.

The ruling by the British Supreme Court blocks an earlier decision by the country’s authorities to cooperate with the U.S. by sharing information about El Shafee Elsheikh and Alexanda Kotey, who is of Ghanaian and Greek Cypriot background.

The British men, captured two years ago by a Kurdish-led, U.S.-backed militia, are accused of participation in a brutal Islamic State group known for beheadings and barbaric treatment of American aid workers, journalists and other hostages in Syria.

The court decision is a setback for the U.S. Justice Department, where officials for years have been investigating the killings. U.S. officials have not announced any charges against the men, but have spoken publicly about their desire to see members of the cell, known as “The Beatles” for their British accents, face justice. The men were transferred to U.S. custody last October as Turkey invaded Syria to attack Kurds who have battling the Islamic State alongside American forces.

“We are disappointed with the UK Supreme Court’s decision and are considering the appropriate next steps,” said Justice Department spokesman Marc Raimondi. “As our investigation of these individuals continues, we will work with our UK counterparts on a path forward, consistent with our shared commitment to ensuring that those who commit acts of terror are held accountable for their crimes.”

It was not clear what those next steps would be, or whether the decision might prompt the Justice Department to remove the possibility of the death penalty from any eventual prosecution. Attorney General William Barr said in a private meeting last year with victims’ relatives that he wanted to see the militants brought to justice.

The U.S. and British governments have an agreement to share documents, records and other evidence in criminal investigations. In 2015, the Justice Department asked for evidence that Britain had gathered on the “Beatles,” saying it was doing its own investigation into Americans who were murdered in Syria.

Though the death penalty has been abolished in the United Kingdom, British authorities were willing to provide their U.S. counterparts with evidence against Elsheikh and Kotey even without assurances that the men would not be executed if convicted.

British authorities said it would not be right to withhold evidence given the horrific nature of the allegations, but some lawmakers called on the government to reserve its position. In July 2018, after lawyers for Elsheikh demanded a review of the decision to allow the men to be put on trial in the U.S., Britain’s Home Office temporarily suspended cooperation with U.S authorities.

Lord Brian Kerr, a judge on the British Supreme Court, said although there was no doubt the crimes the men are accused of are “dreadful” and “of the most awful nature,” it was unlawful to turn over evidence to a foreign country that could be used in pursuit of a death penalty prosecution.

“It follows that no further assistance should be given for the purpose of any proceedings against Mr. El Sheikh in the United States of America without the appropriate death penalty assurances,” Kerr wrote.

The British leader of the “Beatles”‘ cell, Mohammed Emwazi, who was also known as “Jihadi John,” was killed in a 2015 drone strike.