An image of a Catholic priest waving a blood-stained handkerchief as he tried to help a victim to safety on January 30 1972 went around the world.
Former members of the support company of the 1st Battalion Parachute Regiment are facing possible charges from the Public Prosecution Service.
They could include counts of murder, attempted murder and causing grievous injury with intent.
Soldiers had been sent into the Bogside nationalist housing estate to deal with riots which followed a Derry march defying a ban on public processions.
As well as the 13 who died, a total of 15 others were shot and injured. One of the injured died months later from an inoperable tumour and some consider him the 14th fatality.
A public inquiry conducted by a senior judge shortly after the deaths was branded a whitewash by the dead victims’ families and thus began a campaign for a new public inquiry.
Families of the dead sought to right the wrongs of false claims that their loved ones had been armed.
A fresh probe was eventually ordered by former prime minister Tony Blair in 1998.
A decade-long investigation by Lord Saville of Newdigate concluded that the troops killed peaceful protesters and seriously criticised the decision to send them into the Bogside estate in vehicles.
The Saville report used the soldiers’ ciphers to identify some its authors were sure fired lethal shots. In other cases they recorded those who had probably done so.
Following the inquiry’s conclusion in 2010, then prime minister David Cameron said the killings were unjustified and unjustifiable.
An investigation by the Police Service of Northern Ireland (PSNI) followed the £195 million inquiry and files on 18 soldiers were submitted to prosecutors in 2016 and 2017 for consideration. One former soldier has since died.
Four other soldiers included in the Saville Report died before police had completed their investigation.
A decision is also due to be taken on Thursday by the PPS as to whether to charge two Official IRA suspects present on the day.
Papers before prosecutors included 668 witness statements and numerous photos, video and audio evidence.
On Thursday morning the families will gather outside The Museum of Free Derry, just yards from where the killings took place 47 years ago, and march together to a city centre hotel to hear whether charges will be brought.
Alan Barry, from the Justice for Northern Ireland Veterans campaign group, said he feared the paratroopers would face charges, accusing the justice system in Northern Ireland of being one-sided.