Some 90 police pocket notebooks that could provide vital information about the 1989 Hillsborough disaster have been recovered by investigators.

Retired and serving officers handed in the notebooks to South Yorkshire Police, The Independent Police Complaints Commission (IPCC) revealed.

ITV News Correspondent Damon Green reports:

Boxes of notebooks and other documents that cover the period of the disaster were also found by the force and could hold important details.

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.

The IPCC revealed last month that at least one officer made a note of what happened at Hillsborough, against instructions, and none of the previous inquiries had recovered any such notebooks.

The 1989 disaster, that claimed the lives of 96 Liverpool fans, is now at the centre of the biggest ever inquiry into police conduct in the UK.

Director of Public Prosecutions Keir Starmer QC told ITV News he will meet with the families of Hillsborough victims on Friday after they voiced their concerns over the slow progress of the inquiry into the 1989 disaster.

He added that he was "well aware" of their concerns.

Evidence has been uncovered that suggests the statements of 74 more officers might have been changed, and that fans' witness accounts could also have been altered, the IPCC said.

Investigators are set to appeal for witnesses next week in relation to how West Midlands Police ran their inquiry into the handling of the disaster by South Yorkshire Police. Around 12,000 people spoke to West Midlands Police as part of their inquiry.

This appeal, which will be launched next week, forms a crucial element to our investigation into how West Midlands Police conducted its inquiries into the disaster. We want to hear people's experiences of that process. 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.

The appeal is likely to be promoted during Liverpool Football Club's home match on September 21.