Police officers involved in the 1989 Hillsborough disaster may have altered 74 more statements than previously thought, according to the police watchdog.
The Independent Police Complaints Commission (IPCC) also said it has found evidence fans' witness accounts may have been amended as well.
The IPCC added it has been looking through 90 pocket notebooks taken from South Yorkshire Police officers who made notes on the day of the tragedy.
The announcement arrives one year after the Hillsborough independent Panel (HIP) issued its report into the disaster in which 96 people died.
The HIP report found that 164 police statements were significantly amended, with 116 explicitly negative comments about the policing operation removed altogether.
If the latest findings are confirmed, the total number of statements altered by police will be 238.
Deborah Glass, IPCC deputy chair, said: "One year on from the publication of the Panel report it is worth reflecting on the progress so far.
"We have set up and begun an investigation on a scale not previously seen before. Our work has established the foundations for the investigation being conducted by Jon Stoddart's team and the inquest process.
"From our work on this we have recovered West Midlands policy books that have never been seen by previous inquiries [and] recovered pocket notebooks from officers who were on duty on the day of the match.
"We have identified the statements of 74 more officers which may have been amended [and] also uncovered material which would suggest that fans' witness accounts may have been altered.
"We are in the process of interviewing all the surviving officers whose accounts were amended."
The IPCC are also appealing for more witnesses to come forward to help its investigation into how police conducted themselves at the stadium as they hope to produce "the last, definitive account" of the disaster.
In December, the original accidental deaths verdict of those who died was quashed after the report revealed 41 of the 96 victims had the potential to survive.
A new inquest into their deaths is set to begin on 31 March at an unconfirmed location in the North West.