A new PSNI legacy unit has taken over an investigation into the events of Bloody Sunday.
The unit was set up to replace the Historical Enquiries Team (HET).
It will also review other killings that happened before the PSNI’s serious crime branch was established in 2004.
It will re-examine nearly 1,000 killings that happened during the Troubles that were not reviewed by the HET.
Sources say the task could take up to 15 years to complete.
A team of about 40 detectives and civilian staff have started work with the number due to grow to 75 by the end of April.
The PSNI team will operate until a new Historical Investigations Unit proposed in the Stormont House Agreement takes over responsibility for legacy issues.
A fresh police investigation into the 1972 massacre in Derry’s Bogside was opened after calls from families.
However, in November it was revealed that most of the Bloody Sunday investigation team was to be laid off in the wake of £50 million cuts to the policing budget.
British paratroopers opened fire on unarmed civil rights marchers on Bloody Sunday on January 30, 1972. Thirteen people were killed. Fourteen others were wounded, and one later died.
The 2010 Saville Report concluded that soldiers opened fire first and British PM David Cameron subsequently apologised.