Hillsborough survivors have spoken of how they were "scared and intimidated" by officers from the force tasked to independently investigate the football disaster.
The new investigation into the tragedy, which killed 96 Liverpool fans in 1989, has already uncovered a mass of statements given by police officers on the day that were doctored by South Yorkshire Police.
A subsequent inquiry carried out by West Midlands Police into the handling of Hillsborough by their counterparts in South Yorkshire has also become the focus for possible criminal conduct after the Independent Police Complaints Commission found ''differences'' in witness statements taken from fans.
The BBC's Newsnight has spoken to survivors who have also spoken of being mistreated by officers from West Midlands Police and questioned how officers took their statements.
One of them, Nick Braley, said that when he told West Midlands officers that he thought South Yorkshire police failings had caused the disaster, he was told he could face prosecution.
He told the programme he was "scared and intimidated" by a West Midlands officer and was accused of having a "grudge" against police.
Around 12,000 people spoke to West Midlands Police as part of its inquiry.
Analysis shows statements taken by West Midlands Police officers from supporters are not the same as the details given by the same individuals in questionnaires they had completed earlier.
Its investigation formed the basis of Lord Taylor's judicial inquiry into the disaster, the subsequent decision not to prosecute individuals, and the flawed inquests whose verdicts were quashed in 2012.
The IPCC said last year it wants to hear from anyone who gave evidence to West Midlands Police either in writing, via telephone or in face-to-face interviews.
Last September a damning report by the Hillsborough Independent Panel found that South Yorkshire Police orchestrated a cover-up of events, falsified documents and blamed innocent supporters for the tragedy.
It also raised ''serious questions'' over the work done by the West Midlands police force.
The report led to a public apology by Prime Minister David Cameron, the quashing of the original inquest verdicts, the resignation of a chief constable and the launch of two further investigations into the disaster, with criminal prosecutions of individuals and corporate bodies not ruled out.
Last month IPCC investigators said they were close to ''identifying and interviewing people as suspects''.
Britain's worst sporting disaster happened at Sheffield's Hillsborough stadium on April 15 1989 during Liverpool's FA Cup semi-final against Nottingham Forest as thousands of fans were crushed on the ground's Leppings Lane terrace.
Professor Phil Scraton, who played a major role in the independent panel report, told BBC Radio 4's Today programme: "The statement-taking processes adopted by the investigating force, the West Midlands Police, dramatically shows that now the entire evidential base on which the Taylor inquiry, the criminal investigations and the inquest were based is flawed. This is a crucial issue."