The PSNI has said it plans to question witnesses to the killing by British soldiers of 14 Catholic civil rights marchers in Derry 40 years ago.
The interviews form part of a murder investigation announced by the PSNI in 2012 into the Bloody Sunday killings.
On 30 January 1972, British troops opened fire during an unauthorised march in the Bogside in Co Derry.
Soldiers killed 13 people and injured 14, one of whom died later. The victims were all unarmed Catholics.
A spokesperson for the PSNI said: "Preliminary work has begun into what will be a lengthy and complex investigation.
"For the investigation to be as comprehensive and effective as possible, police will be asking for public support in the form of witnesses who gave evidence to the Saville Inquiry now making statements to detectives."
That inquiry by High Court judge Lord Saville was the longest and most expensive in British legal history.
It concluded in 2010 that there was no justification for the shootings, prompting Prime Minister David Cameron to apologise for the killings.
Citing an unnamed source close to the police, the Sunday Times newspaper said up to 20 retired British soldiers could be arrested on suspicion of murder or attempted murder.
A spokesman for Britain's Ministry of Defence declined to comment.