Former soldiers will find out later if they are to stand trial over the killing of 13 people on Bloody Sunday.
The Public Prosecution Service is due to make its decisions public at 11:00 GMT, having weighed up 125,000 pages of material.
Charges, including murder and attempted murder, have been considered against 17 British Army veterans.
In 2010, a public inquiry under Lord Saville concluded the 1972 shootings in Londonderry were unjustified.
A number of relatives of those killed walked from the Museum of Free Derry to the city centre on Wednesday ahead of being told the PPS decision.
Bloody Sunday is one of the darkest days of the Northern Ireland Troubles.
Thirteen people were killed and 15 injured on 30 January 1972, when troops fired more than 100 times as trouble broke out at a civil rights march.
One of the injured died five months later but the Saville Inquiry said it was “not the result of any of the wounds he sustained on Bloody Sunday”.
Families of the victims will be told if any of the former soldiers, now in their 60s and 70s, are to face charges shortly before the news is made public.
Around the same time, the decisions will also be communicated to solicitors representing the former Parachute Regiment soldiers.
John Kelly, whose brother Michael was one of those killed, told BBC News NI he was “nervous” in anticipation of the decisions.
“It is the last part of the journey we are on. It would be fantastic to achieve justice for our loved ones.”
Lord Saville’s 5,000-page report stated none of the casualties was posing a threat of causing death or serious injury and that soldiers had lost their self-control.
The prime minister at the time of the report’s publication, David Cameron, apologised for the soldiers’ conduct.
A police investigation into Bloody Sunday followed Lord Saville’s 12-year, £200m public inquiry.
A file was sent to the Public Prosecution Service (PPS) in November 2016.
The PPS has also been considering whether any charges should be brought against two former members of the Official IRA over its actions on Bloody Sunday.
The Saville Inquiry heard evidence of shots being directed at troops, but “on balance” a soldier had fired first on the day.
In total, police reported 20 suspects to the PPS – 18 of them former soldiers, one of whom died last year.
The file contains 668 witness statements, as well as photographs, film and audio material.
Prosecutors firstly assessed whether evidence was sufficient to provide a reasonable prospect of convictions and if so, whether prosecutions were in the public interest.