Two former members of the Official IRA could face prosecution for their alleged role in the events of Bloody Sunday.
Prosecutors are already examining files of evidence against 18 ex-British soldiers who were on the streets of Derry in 1972 when the Parachute Regiment shot and fatally wounded 14 civil rights demonstrators.
A public inquiry into the killings heard evidence that Official IRA members fired shots at soldiers in Derry during the day.
The PSNI has now sent a file on one former Official IRA member to Northern Ireland's Public Prosecution Service (PPS) for consideration and is preparing a file on another ex-member.
Those will be considered along with the files on the 18 ex-British Paras. A decision on whether any of them will face prosecution is expected within six months.
The PPS has written to the families of those killed to inform them of the development.
In 1969 the IRA spilt into the Official IRA and Provisional IRA.
While the Provisional movement went on to wage an intensive armed campaign throughout the Troubles, the Officials did not engage in violence on the same scale.
Thirteen people were shot dead by soldiers on Bloody Sunday. A 14th died four months later in hospital.
A murder investigation was launched in 2012.
It was initiated after the government-commissioned inquiry, undertaken by Lord Saville, found none of the victims was posing a threat to soldiers when they were shot.
Following the publication of the Saville report in 2010, then British prime minister David Cameron apologised for the British Army's actions, branding them "unjustified and unjustifiable".
A PPS spokesman said: "We have provided regular updates to all of the families in relation to the Bloody Sunday case, with the most recent contact last week.
"Over the course of last year, the PSNI has provided around 35 lever arch files of material, with further material anticipated.
"Our focus currently is completing our analysis of the police investigation files and on the identification of all significant legal issues and their resolution.
"We have consistently made it clear that the prosecution task in this case is considerable but we are making steady progress.
"We can confirm that one further suspect has been added to the case recently with a further report expected.
"However, it is not anticipated this will materially affect our timescale for decision-making."