Nineteen people have refused to assist with an independent probe into West Midlands Police's role after the Hillsborough disaster.
The Independent Police Complaints Commission (IPCC) has contacted 258 officers and staff members who were involved with the force's investigation into the 1989 tragedy which killed 96 Liverpool fans.
A total of 161 statements have been taken so far but 19 individuals declined to co-operate with the IPCC and nine people were unable to assist for health reasons, said the watchdog.
The role of West Midlands Police (WMP) and those who led its investigation is one of numerous areas the IPCC is looking at in its largest-ever inquiry into alleged criminality and alleged police misconduct.
An investigation team of WMP was tasked with investigating South Yorkshire officers' conduct for the initial independent inquiry led by Lord Chief Justice Taylor, the Director of Public Prosecutions (DPP) and the South Yorkshire Coroner.
The IPCC is reviewing and analysing thousands of documents to help establish how that evidence was gathered and then handled, including what was provided to the DPP in 1990.
It said it continued to look at whether there was any influence placed under the then coroner Dr Stefan Popper by police officers and was trying to identify and trace coroner's officers from the period.
Another ongoing line of inquiry was assessing what was known and understood by WMP in relation to the amended accounts of South Yorkshire Police officers.
In its latest overall monthly update into the probe, the IPCC said it also continued to liaise closely with the Crown Prosecution Service (CPS) about those individuals it considered to be suspects and had submitted "substantial evidence-based files".
In April, an inquest jury concluded the 96 victims were unlawfully killed and that blunders by South Yorkshire Police "caused or contributed to" the disaster at Sheffield Wednesday FC's stadium.