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.
An IPCC spokesman said: "We can confirm the Independent Police Complaints Commission is managing an investigation into allegations against a number of former and serving Staffordshire Police officers.
"The investigation is being carried out by the Chief Constable of Derbyshire, Mick Creedon, under the direction and control of the IPCC.
"His investigation began following a request from the Criminal Cases Review Commission who are conducting an inquiry on behalf of the Court of Appeal in relation to an ongoing appeal.
"Subsequently the chief constable's investigation raised matters which were referred to the IPCC.
"As matters are sub-judice pending the appeal case we cannot provide further information at present."
A spokesman for Northamptonshire Police Authority said: "Northamptonshire Police Authority is aware that the IPCC is managing an investigation into allegations against a number of former and serving Staffordshire Police officers, which relate to the handling of a criminal case from several years ago in Staffordshire.
"We can confirm that allegations have been made in relation to Chief Constable Adrian Lee and Deputy Chief Constable Suzette Davenport, who both served in Staffordshire Police.
"The authority has considered information provided by the IPCC and remains completely confident in both the chief constable and deputy chief constable.
"We wish them to continue driving the force forward in these challenging times and their focus, along with that of the authority, remains on building upon the significant successes of last year in improving force performance, managing the budget gap, and making Northamptonshire an even safer county."
A Staffordshire Police Authority spokesman stressed that the notices of investigation were "not judgmental in any way" and did not indicate wrongdoing.
He said: "We can confirm that a serving chief officer has been served with a notice advising them that their conduct is subject to investigation.
"Such notices are not judgmental in any way and we need to let the ongoing investigation run its course and establish the facts.
"As a result, the Police Authority has taken the decision not to suspend the officer.
"Due to the ongoing IPCC inquiry and high court appeal, it would be entirely wrong to comment in any further detail on the case.
"The force and authority are continuing to fully co-operate with the IPCC and its investigation team."
West Midlands Police Authority was not immediately available for comment.
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.
A CCRC spokesman said: "I confirm that the Criminal Cases Review Commission is investigating matters on behalf of the court of appeal in relation to an ongoing appeal involving Joof and others."