The Northern Ireland Director of Public Prosecutions was not involved in the police decision to launch a murder investigation into the Bloody Sunday killings, his office said.

Barra McGrory, QC, represented Martin McGuinness in the Saville Inquiry into the shootings in Londonderry in January 1972 when the Sinn Fein MP was an IRA leader in the city.

A report which confirmed the innocence of the 13 men shot dead by British Paratroopers also claimed that Mr McGuinness, now the deputy Northern Ireland First Minister, gave wrong information about his movements that day and was probably carrying a machine gun which he may have used to open fire on troops. A 14th victim died later.

The chief constable of the PSNI, Matt Baggott, said the investigation involving up to 40 officers could take four years to complete.

Mr McGrory insisted he had no part in the decision by the chief constable to begin a murder inquiry.

A spokesman said when he took up his position as director last November he had identified Bloody Sunday as one of a number of cases in which there may be a potential conflict of interest.

Some Unionist politicians, furious with the chief constable's decision, are demanding Mr McGuinness be questioned and the inquiry be widened to include the murders of two RUC officers shot dead by the IRA in Derry just days before Bloody Sunday.

East Londonderry DUP MP Gregory Campbell said: "If the material contained in the Saville Report is good enough to warrant an investigation of the soldiers, then the police will also note that the report indicates the Deputy First Minister was 'probably' carrying a sub-machine-gun on that day. This must also merit investigation by the police."