Suspected drug dealer Kevin Nunes was shot dead in a
country lane in Staffordshire
The Crown Prosecution Service (CPS) is to consider charging 14 serving and former police officers, including two current chief constables, over their roles in a murder investigation.
The Independent Police Complaints Commission (IPCC) looked at how the officers acted before the trial of five men over the death of Kevin Nunes.
Seven of the officers involved have retired from Staffordshire Police.
Mr Nunes was shot dead in Pattingham, Staffordshire, in September 2002.
The IPCC investigation examined the officers' dealings with a protected witness and disclosure issues before the trial in 2008.
The four most senior-ranking officers implicated are:
They were all senior officers at Staffordshire Police at the time of the allegations.
In March 2012 five West Midlands men who were jailed in 2008 for the murder were freed on appeal, after it emerged concerns over the credibility of the key prosecution witness were not disclosed to the defence.
Adrian Lee, Marcus Beale, Suzette Davenport and Jane
Sawyers are being investigated by the IPCC
Police believe Mr Nunes was a drug dealer who was shot in an execution-style killing after straying into another gang's territory.
The IPCC has reviewed more than 8,000 documents and taken about 300 statements as part of its investigation and interviewed each of the 14 officers at least twice.
IPCC commissioner Tom Davies said: "This has been a thorough and painstaking investigation which has required careful scrutiny of thousands of documents and hundreds of statements, and interviews have been carried out with relevant officers."
In a statement, Staffordshire Police said: "We've been made aware of today's development and now await the outcome of the CPS review. We will continue to fully co-operate with the IPCC-led investigation."
Evidence has been passed to the CPS relating to five officers, up to the rank of detective chief inspector, who were part of the Staffordshire Police sensitive policing unit responsible for handling protected witnesses, the IPCC said.
It added that three of the five officers are retired from the force.
The IPCC said a file would be sent to the CPS in coming weeks about a further nine officers, of the rank of superintendent and above, who held senior positions at the force.
Four of those are serving at the rank of assistant chief constable or above, while four of the nine officers are retired from Staffordshire Police.
Levi Walker, from Birmingham, Adam Joof, from Willenhall, West Midlands, Antonio Christie, from Great Bridge, West Midlands, Michael Osbourne and Owen Crooks, both from Wolverhampton, all had their murder convictions quashed last year after being found guilty at Leicester Crown Court in 2008.