The UK has sent the US evidence on two British alleged Islamic State (IS) militants, clearing way for a trial.
Alexanda Kotey and El Shafee Elsheikh are accused of belonging to an IS cell dubbed The Beatles, who were behind the killings of Western hostages.
It comes after Britain’s High Court rejected the request by the mother of one of the suspects to block the transfer of information.
The men, in US military custody in Iraq, deny the charges.
The pair are alleged to have been members of an IS gang responsible for the death of hostages in Iraq and Syria in 2014.
The victims – who included American journalists and British aid workers – were beheaded and their deaths filmed and broadcast on social media.
The US sought the UK’s help in the case but until recently a legal fight over the use of the death penalty stymied co-operation.
Last month, the US made clear the two would not be executed if found guilty.
IS once controlled 88,000 sq km (34,000 sq miles) of territory stretching from western Syria to eastern Iraq. It imposed its brutal rule on almost eight million people.
What did the UK say?
On Thursday, Home Secretary Priti Patel confirmed that the UK had forwarded the evidence it had against the two suspects to the US.
Pleased to say that the further evidence to support the prosecution of Kotey & El Sheikh has now finally been transferred to the US. I sincerely hope that justice for the victims and their families will now be served, Ms Patel wrote in a tweet.
The mother of one of the suspects had earlier won her legal battle to prevent any UK evidence from being used against the accused in a US court. But Tuesday’s High Court ruling lifted that ban.
Among the victims was British photojournalist John Cantlie – who was kidnapped in 2012 in Syria along with US reporter James Foley.
Cantlie’s sister, Jessica Pocock, told the BBC’s Frank Gardner of her relief that those accused will be held to account.
We’ve only ever wanted these two to face justice, she said.
The families of all the American and all the British people who were taken, so that’s David Haines, Alan Henning and John Cantlieā¦ they need nothing less than a fair trial, she said.