The brother of a victim of the ISIS gang known as the Beatles said he will give evidence to support their prosecution in the US.
Mike Haines, brother of aid worker David who was beheaded on camera in September 2014, gave a statement to British police and is ready to travel to the US if summoned.
El Shafee Elsheikh, 32, and Alexanda Kotey, 36, from West London, appeared by videolink in a US court on Friday, accused of four murders connected to their membership of a group of four ISIS executioners.
Each charge could carry a life term in solitary confinement at a maximum-security prison in Colorado known as the Alcatraz of the Rockies.
Last week they were secretly flown from a US military base in Iraq to an undisclosed location in the US. The men will not face the death penalty under a deal made in the UK.
David Haines, of Yorkshire, North England, was kidnapped by ISIS in early 2013 while working in a Syrian camp for internally displaced people.
Mike Haines told The Times newspaper that the kidnappers did not respond to his family when they tried to get in touch.
He believed they never intended to release their British and American hostages.
The family did receive a letter from David, written on his kidnappers' behalf, when a French captive was released in 2014.
It demanded €150 million ($177.3m).
“If every member of the family had sold everything, we wouldn’t have come close," Mr Haines said.
"I thought of every different option, even a bank robbery."
He said he would forgive his brother’s killers if they were to apologise.
“I try not to have anger, I try not to have hate," Mr Haines said. "Every day I have a daily battle with hate but if I do hate, I am not honouring my brother."
On Sunday, it emerged that Elsheikh and Kotey are assembling a top legal team, expected to be funded by the public purse.
It includes a top US lawyer who represented the 9/11 terrorists and one who acted for Rose McGowan, the first actress to accuse the Hollywood producer Harvey Weinstein of rape.
On the team is terrorism expert Edward MacMahon, who travelled to Guantanamo Bay to represent Walid bin Attash, one of the main planners of the September 11 attacks in 2001.
Bin Attash also orchestrated the bombing of the USS Cole warship in Yemen the year before.
The Beatles was an assassination squad of four ISIS terrorists who were named after the rock band because of their British accents.
The group included Mohammed Emwazi, known as "Jihadi John", who was killed in a 2015 drone strike.
The fourth member, Aine Lesley Davis, was sentenced to seven years in prison in Turkey in 2017.
They guarded at least 23 western ISIS hostages in cramped cells in western Raqqa. Nearly all of the captives were ransomed or killed.
Among the hostages they executed were American journalists James Foley and Steven Sotloff.
Elsheikh and Kotey were arrested in January 2018 by Syrian Democratic Forces as they tried to escape Syria for Turkey, as ISIS lost territory.