A five-strong police ombudsman team will interview a series of witnesses of the Kingsmills massacre.
Ten Protestant workmen were shot dead by the IRA in 1976 as they drove home near Kingsmills in south Armagh, in what has been described as the Protestant Bloody Sunday.
In February the Police Ombudsman for Northern Ireland (PONI) confirmed it had reviewed the murder file from the 1970s and the related HET report, after complaints from relatives about the police investigation into the atrocity.
A spokesman said previously: “We have now satisfied ourselves that the outstanding issues are matters which the police ombudsman can and should investigate.
“That investigation will focus on an allegation that the police investigation of the Kingsmills attack was poor, that from an early stage police did not secure the murder scene and that they later failed to arrest named suspects.”
Two suspects hold on-the-run comfort letters.
PONI confirmed that a senior investigator, his deputy and other members of his team visited the offices of Markethill victims’ group FAIR on Wednesday.
“The purpose of the meeting was to introduce some of the families to the investigators who would be working on this case,” the spokesman said.
The investigation will look at matters related to the murders of Bertie Frazer, James Hunter and the killings at Kingsmills.
He added: “The first stages will include a review of the relevant material in the public domain and all the available police information, including intelligence material.
“We will then move to interview a series of witnesses. The reports of a palm print on the minibus will be among the issues the team will consider.”
Work is due to begin shortly, pending any matters which may arise at the preliminary inquest hearing on Kingsmills next month. The inquest proper is expected to open after Easter 2016.
Victims’ campaigner Willie Frazer said Wednesday was an historic day.
“For the first time in history in Northern Ireland an inquest is taking place which also has an active investigation running alongside,” he said. “The only other time this has been known to happen in the UK was with the Hillsborough disaster.”
He said that the Tullyvallen massacre – only months before Kingsmills – and the murder of his father Bertie Frazer would also be included in the investigations, meaning some 30 murders would be reviewed for linkages.
The PSNI said it would be inappropriate to comment.