Police officers could face criminal charges over alleged changes to statements after the Hillsborough disaster, it has been announced.
The Independent Police Complaints Commission (IPCC) said a "large number" of current and former officers would be investigated over what happened on the day of the tragedy in Sheffield in 1989 - and during the subsequent alleged cover-up.
Director of Public Prosecutions Keir Starmer said prosecutors would consider if there was enough evidence to charge any individual or corporate body with any criminal offence.
The IPCC's deputy chair Deborah Glass said: "I think I can confidently say this will be the largest independent inquiry that has been launched into the actions of the police in the United Kingdom."
Last month, a report by the Hillsborough Independent Panel claimed a shocking cover-up was staged in order to shift blame onto the 96 Liverpool fans who died following the crush during the FA Cup semi-final match against Nottingham Forest.
It alleged that 164 police statements were altered in the wake of the tragedy, 116 of them to remove or change negative comments about the policing of the match and the ensuing disaster.
Lawyers for some of the victims' families have argued that corporate manslaughter charges should be levelled against South Yorkshire Police, Sheffield City Council, Sheffield Wednesday and the Football Association.
South Yorkshire Police dealt with the initial tragedy and West Midlands Police investigated how South Yorkshire handled the disaster.
Ms Glass said: "The potential criminal and misconduct offences disclosed by the panel's report fall into two broad categories.
"They are the allegations that go to the heart of what happened at Hillsborough in April 1989 and individuals and institutions may be culpable for the deaths, and there are allegations about what happened after the disaster, that evidence was fabricated and misinformation was spread in an attempt to shift blame.
"We will investigate the role of South Yorkshire Police and West Midlands Police in these matters. This will mean that a large number of current and former officers will be under investigation, including Sir Norman Bettison, whose conduct was referred by the West Yorkshire Police Authority."
Following the publication of the report, a complaint was made to the IPCC that Sir Norman, who was a chief inspector in South Yorkshire at the time, had supplied misleading information in the wake of the disaster.
A second element of the complaint was over comments made by Sir Norman, currently Chief Constable of West Yorkshire Police , after the report was published.
He said that Liverpool fans' behaviour made policing the tragedy "harder than it needed to be", sparking furious calls for him to resign.
It also emerged that Sir Norman will face a further investigation by the IPCC over claims he tried to influence their decision-making process in relation to the Hillsborough allegations.
Ms Glass said: "This alleges that he attempted to influence the decision-making process of the authority, that is the West Yorkshire Police Authority, in connection with the referral they made on September 18."
Sir Norman announced last week that he would retire in March, saying: "Recent weeks have caused me to reflect on what is best for the future of policing in West Yorkshire and I have now decided to set a firm date for my retirement." He has welcomed the investigation.
Margaret Aspinall, chairwoman of the Hillsborough Families Support Group , told Sky News: "At this moment in time, I'm very, very happy.
"I think the families will be pleased to hear (this). They're going to look back into all the serving policemen on that day and I think that's a great thing to do, because there were so many involved, I think, in the heart of that cover-up."
Liverpool FC welcomed the announcement. Managing director Ian Ayre said: "This is another significant step forward in the campaign for justice for Hillsborough families and survivors."