A High Court action by seven former paratroopers who are facing questioning over the Bloody Sunday shootings will have its first public hearing today.
The ex-soldiers are applying for permission to seek a judicial review into the way the Police Service of Northern Ireland (PSNI) is conducting its historical probe into the deaths of 14 civil rights demonstrators in Londonderry in 1972.
They are expected to argue they should not be arrested and taken to Northern Ireland for questioning because they are willing to voluntarily attend police interview in England.
The legal challenge was lodged with the High Court in London after the arrest of a former colleague in Northern Ireland - the first ex-soldier detained.
The arrest of the 66-year-old, who was held in Co Antrim and later released on police bail, was welcomed by relatives of those killed.
It is understood the PSNI has agreed not to make any further arrests until the courts decide whether or not to uphold the challenge of the seven.
The first public hearing is before the Lord Chief Justice, Lord Thomas, and two other judges.
A petition calling for soldiers involved in Bloody Sunday to be granted immunity from prosecution has gained tens of thousands of supporters.
Thirteen people were killed by members of the Parachute Regiment on the day of the incident in Derry's Bogside. Another victim of the shootings died in hospital four months later.
Northern Ireland police launched the murder investigation in 2012. It was initiated after a Government-commissioned inquiry, undertaken by Lord Saville, found that none of the victims was posing a threat to soldiers when they were shot.
Following the publication of the Saville report in 2010, Prime Minister David Cameron apologised for the Army's actions, branding them "unjustified and unjustifiable".
In September, the PSNI told bereaved families they intended to interview a number of former soldiers about their involvement on the day.