Four senior police officers, including a chief constable and his deputy, are being investigated over claims of misconduct relating to a gangland murder case.
The Independent Police Complaints Commission's (IPCC) probe relates to the handling of an investigation by Staffordshire Police into the murder of amateur footballer Kevin Nunes, 20, who was gunned down in a country lane in 2002. Five men were jailed in connection with the killing in 2008.
The IPCC confirmed that formal notice of investigation had been served on "a number of former and serving Staffordshire Police officers".
It would not confirm reports in The Guardian that the allegations related to concerns over potentially relevant evidence being withheld from the prosecution in the trial.
Northamptonshire Police Authority confirmed that its force's chief constable Adrian Lee and deputy chief constable Suzette Davenport were being investigated. Mr Lee is also the head of the Association of Chief Police Officers' ethics portfolio.
The other two senior officers involved in the probe are understood to be Jane Sawyers, assistant chief constable at Staffordshire Police, and Marcus Beale, assistant chief constable at West Midlands Police.
The IPCC investigation was launched after the men convicted of Nunes' killing lodged an appeal with the Court of Appeal, which asked the Criminal Cases Review Commission (CCRC) to investigate. The CCRC subsequently referred the case to the IPCC.
Nunes, a drug dealer who had been on the books of Tottenham Hotspur, was shot dead in a an execution style killing after a gang dispute. His body was found with five gunshot wounds in a country lane in Pattingham, Staffordshire, on September 19, 2002.
His killers - Levi Walker, from Birmingham; Antonio Christie, from Great Bridge, West Midlands; Adam Joof, from Willenhall, West Midlands; Michael Osbourne, and Owen Crooks, both from Wolverhampton, were all jailed for life after being found guilty of murder by a jury at Leicester Crown Court.
Walker, Christie and Osbourne were all 28 when they were jailed, Joof was 27 and Crooks was 26.