Four senior police officers - including Northamptonshire's chief constable and deputy chief constable - are being investigated over misconduct claims.
The Independent Police Complaints Commission inquiry relates to Staffordshire Police's investigation into Kevin Nunes' 2002 gangland murder.
It is claimed the four, who are either former or serving Staffordshire Police officers, withheld potential evidence.
All four senior officers remain in their posts.
Northamptonshire Chief Constable Adrian Lee and Deputy Chief Constable Suzette Davenport are being investigated, along with West Midlands Assistant Chief Constable Marcus Beale and Staffordshire's Assistant Chief Constable Jane Sawyers.
Five men were jailed in 2008 in connection with the death of amateur footballer Kevin Nunes who was shot dead in a country lane in Pattingham, Staffordshire, in September 2002.
The 20-year-old drug dealer was on the books of Tottenham Hotspur and was shot in an execution-style killing after cutting in on a gang's lucrative trade moving crack cocaine from Wolverhampton to Aberdeen.
The IPCC investigation into the handling of the murder inquiry started after the convicted men lodged an appeal with the Court of Appeal, which asked the Criminal Cases Review Commission to investigate.
Last year, the IPCC was brought in amid concerns evidence may have been withheld from the prosecution and the court.
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."
A spokesman for Northamptonshire Police Authority said: "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."
Mr Lee joined Northamptonshire Police from the Staffordshire force in 2009. He is also the head of the Association of Chief Police Officers' ethics portfolio.
A Staffordshire Police Authority spokesman confirmed the conduct of one of its serving officers was being looked at but stressed that the notices of investigation did not indicate wrongdoing.
He said: "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.
"The force and authority are continuing to fully co-operate with the IPCC and its investigation team."
Five men were jailed for life after being found guilty of Mr Nunes' murder by a jury at Leicester Crown Court in 2008.
They were Adam Joof, of Derwent Close, Willenhall, West Midlands; Antonio Christie, of Devey Drive, Great Bridge, West Midlands; Levi Walker from Birmingham; Michael Osbourne, of Hobgate Road, Heath Town, Wolverhampton; and Owen Crooks from Wolverhampton.
Walker, Christie and Osbourne were all 28 years old when they were jailed, Joof was 27 and Crooks was 26.
They were given minimum tariffs of between 25 and 28 years.