Witness statements from fans present at the Hillsborough disaster may have been altered by police, according to a body investigating allegations of police corruption.

The Independent Police Complaints Commission (IPCC) watchdog said it had unearthed boxes of documents covering the period of the April 1989 tragedy, as well as 90 police pocket notebooks handed in by retired and serving officers.

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. It was the worst sporting disaster in British history.

An independent report released in September last year said the local force, South Yorkshire Police, had tried to cover up what had happened, prompting an apology from Prime Minister David Cameron.

The report said police had changed 164 statements relating to the disaster but recent findings by the IPCC suggest that the statements of 74 more officers might have also been altered.

"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," said IPCC deputy chairwoman Deborah Glass.

"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."

None of the previous inquiries into the tragedy had recovered any police notebooks.

Officers present on the day of the disaster were instructed by their superiors not to make notes about what had happened in their notebooks but the IPCC said last month that at least one officer had disobeyed the instructions.

The IPCC's probe is the biggest ever investigation of police conduct in Britain. It is due to issue an appeal for witnesses next week.

Meanwhile, a senior detective leading a separate investigation into the tragedy says that South Yorkshire Police, Sheffield Wednesday Football Club, Sheffield City Council and the Football Association are being investigated for possible criminal culpability.

Former chief constable Jon Stoddart told the Guardian newspaper: "We are exploring all liability, both public and individual. We are looking at unlawful killing; who is responsible for the deaths."

He said that the readiness of the Hillsborough stadium, the home of Sheffield Wednesday, to host such a match would also be scrutinised.

"It is about the safety of the stadium, certification, the planning and preparation, the engineering and design that went into the Leppings Lane end (where the crush occurred)," he said.

The Guardian said that the stadium's safety certificate was 10 years out of date, that the council was statutorily responsible for licensing the stadium as safe and that the FA approved the ground despite safety breaches and previous, non-fatal crushes there at FA Cup semi-finals in 1981, 1987 and 1988.

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