500 Hillsborough witnesses react to appeal
A child looks at the Hillsborough Memorial at Liverpool FC's Anfield football ground on April 15, 2013.
The Independent Police Complaints Commission (IPCC) launched the appeal on Tuesday as part of an investigation into police handling of the tragedy, in which 96 Liverpool fans died.
The probe has already unearthed evidence that the local police force, South Yorkshire Police, altered statements submitted by officers on the day of the disaster.
On top of that, the IPCC has found evidence suggesting that in a subsequent inquiry, carried out by police officers from the West Midlands, statements from supporters present on the day were also doctored.
An estimated 12,000 people were interviewed by West Midlands Police during its inquiry and the IPCC says it wants to hear from people who spoke to the force, as well as anyone who has not spoken to any previous investigating teams.
"We have had a very encouraging initial response to the appeal," said IPCC deputy chairwoman Deborah Glass.
"We want to speak to as many people as possible and I would urge people to contact us. We want to build the most comprehensive account there has ever been about the events of Hillsborough and we can only do that with the help of those who were there on the day and those who gave accounts previously to West Midlands Police."
She added that the IPCC also wanted to trace people present on the day of the tragedy who have since moved abroad, saying: "This is a national and international search for witnesses. We are investigating events from more than 24 years ago and in that time people have moved, sometimes overseas.
"We want to reach out to them. We want to ensure everyone has a voice in our investigation."
The disaster saw 96 Liverpool fans crushed to death amid overcrowding at one end of Hillsborough Stadium in Sheffield, northern England, before an FA Cup semi-final between Liverpool and Nottingham Forest in April 1989. It was the worst sporting disaster in British history.
An independent report released in September last year said that South Yorkshire Police had tried to cover up what had happened, prompting an apology from Prime Minister David Cameron and the opening of two new investigations.
The report also raised "serious questions" about the investigation carried out by West Midlands Police.