West Midlands police may be responsible, IPCC says, and new evidence has been found from 90 South Yorkshire pocket books

Witness statements from fans who saw the Hillsborough disaster unfold may have been altered by officers from West Midlands police, according to investigators carrying out the biggest police corruption inquiry ever mounted in England and Wales.

The Independent Police Complaints Commission said its inquiry into the aftermath of the stadium tragedy, which killed 96 Liverpool fans, has also uncovered new evidence, with the discovery of 90 pocket books written by South Yorkshire officers on duty at the time and never before located.

More than 100 investigators and staff are involved in the IPCC inquiry into the alleged black propaganda campaign mounted by South Yorkshire police following the disaster in April 1989. A separate criminal investigation into all agencies involved, led by former Durham chief constable Jon Stoddart, is being run alongside the police watchdog's work.

Speaking on the first anniversary of the highly critical Hillsborough panel report – which led to the setting-up of the dual criminal investigations – Deborah Glass, deputy chair of the IPCC, revealed that the watchdog had uncovered 74 more police statements that appeared to have been altered. The panel reported last year that 164 police statements had been amended; of those, 116 were either removed or changed to alter negative comments about the police operation at the FA Cup semi-final between Liverpool and Nottingham Forest.

The latest developments mean that at least 238 statements made by police in the aftermath of the stadium disaster could have been altered.

Glass also revealed that witness statements from fans who were at Hillsborough might have been amended by West Midlands police, the force which investigated South Yorkshire officers' conduct after the 1989 deaths.

The IPCC is to mount an appeal next week for witnesses to the disaster to come forward and wants to speak to anyone who thinks their statements were altered.

Glass said: "From our work … we have recovered West Midlands policy books that have never been seen by previous inquiries. We have identified that the statements of 74 more officers may have been amended. We have also uncovered material which would suggest that fans' witness accounts may have been altered. We have recovered pocket notebooks from officers who were on duty on the day of the match. We are in the process of interviewing all the surviving officers whose accounts were amended.

"This is an ongoing criminal investigation the like of which has never been seen before in this country … Already we are uncovering more about the disaster and its aftermath. Hillsborough has had a history of inquiries by the police and others, many completed quickly, coming to flawed conclusions. Our investigations need to deliver the last, definitive account."

Glass said the witness appeal, to be launched next week, was being supported by Liverpool FC, which would be broadcasting it at its game on 21 September. The IPCC says the witness appeal is a crucial element of its investigation into how West Midlands police conducted its enquiries into the disaster.

"We want to hear people's experiences of that process,"Glass added.

"We have already had a number of people contact us with concerns that their statements were amended and we have no doubt there are others who have not contacted us: we want to be able to present as full as possible a picture of witness evidence both for the inquests and the criminal investigations."

Interviews with officers whose statements have been amended have begun, Glass said. So far three interviews have been carried out and 11 have been scheduled. The IPCC said the interviews should be completed by the end of October and that serving and retired officers were assisting the investigation.

The discovery of the pocket books written by South Yorkshire officers is a major breakthrough. All officers on the day were told to make detailed accounts of their experiences in separate documents and not their pocket books, but a number of serving and retired officers have come forward to say they wrote notes in their pocket books, and have produced them for the inquiry after an appeal was put out by the South Yorkshire force.