More than 100 police officers and staff have not assisted an investigation into the Hillsborough disaster, the police watchdog has said.
The Independent Police Complaints Commission (IPCC) is investigating possible offences committed in the aftermath.
Of the 132 of those yet to respond, 64 are from South Yorkshire Police and 68 from West Midlands Police.
Ninety-six Liverpool fans died as a result of the crush in April 1989.
In her latest update, IPCC Deputy Chair Rachel Cerfontyne said attempts to contact those “who have not assisted our investigation so far” was ongoing.
She added: “Some were unable to provide an account for reasons such as poor health. Others have not responded to our contact and there are a number of individuals who couldn’t be traced.”
The potential witnesses had been identified because they may have useful information and further efforts would be made “to persuade this group to engage with us”, she said.
To date, the IPCC said it has recorded over 4,000 witness accounts, with about 1,200 of those coming from police staff.
But while the watchdog can compel police witnesses who are still serving to give evidence it cannot compel retired officers to do so, Ms Cerfontyne said.
A separate criminal investigation, Operation Resolve, is looking into the crush and the match day itself,
A spokesman for Operation Resolve said the appeal had resulted in 41 possible identifications for 16 of the people.
He said those sought were not suspected of wrongdoing but may have helpful information.
The 19 people police want to trace were seen on CCTV at about 14:50 before the FA Cup semi-final, at the stadium’s Leppings Lane end near exit gate C.
In April, the jury at the inquests into the deaths of the 96 Hillsborough victims concluded they were unlawfully killed.
Operation Resolve is one of two criminal investigations ordered after the Hillsborough Independent Panel’s report in 2012.