More police statements relating to the Hillsborough disaster "appear to have been amended", the Independent Police Complaints Commission (IPCC) has said.
The IPCC investigation is looking at the deaths of the 96 Liverpool football fans in April 1989.
Investigators initially found 164 police statements had been altered.
However, the IPCC says it has now "identified an additional 55 officers whose statements appear to have been amended."
An investigation by the Hillsborough Independent Panel showed police and emergency services had made "strenuous attempts" to deflect the blame for the disaster on to the fans.
Of the altered statements previously identified, 116 of them had negative comments about the policing of the match removed or changed.
'Tip of the iceberg'
Deborah Glass, deputy chair of the IPCC said: "We will begin interviewing those officers [who gave the 164 altered statements] this month. In addition, we have identified more statements that have been altered."
The work to recover and analyse documents is ongoing and the IPCC has now obtained 97% of the documents seen by the panel.
The IPCC's Hillsborough Contact team has received 230 pieces of correspondence since October and more than 50 relate to statements.
Ms Glass said: "We know the people who have contacted us are the tip of the iceberg.
"Preparations are ongoing for an appeal for witnesses to the disaster and this is expected to be conducted in the autumn."
Welfare support will be offered to those who recount the events at Hillsborough as part of the IPCC investigation.
The watchdog will review allegations surrounding amendments to statements, the actions of the police officers after the disaster and how West Midlands Police investigated South Yorkshire Police's conduct.
A separate team, led by former Durham Chief Constable Jon Stoddart, is investigating the response of all other agencies involved in the aftermath of the disaster.
Inquests into the deaths of the 96 victims are set to begin by 31 March at an unconfirmed location in the north west.
Verdicts of accidental death from the original inquest in March 1991 were quashed by the High Court last December.