Britain's Independent Police Complaints Commission has initiated a criminal investigation into police misconduct in a 1989 tragedy at a soccer ground where 96 people died.
The investigation follows a private inquiry that found serious failings by police and emergency services.
The private report, released last month, reviewed thousands of documents and cast doubt on the original inquest's finding of accidental death.
The crush at Sheffield's Hillsborough Stadium on April 15, 1989, has cast a lasting shadow over Liverpool and the surrounding Merseyside area.
The panel that compiled the private report also found evidence that indicated as many as 41 of those crushed could have survived.
The Independent Police Complaints Commission, which oversees police complaints in England and Wales, said Friday that last month's report "revealed extremely serious and troubling issues for the police."
"Its contents provoked a demand for those responsible for the actions revealed in the report to be held to account," it said.
The commission is reviewing hundreds of thousands of pages of evidence, and is paying special attention to allegations that police tampered with statements and that misleading information was passed to the media, among others.
"We have learned details of the run-up to the disaster including the unheeded warnings from previous incidents, the disaster itself, and its aftermath, including what appear to be attempts to distort the truth," the commission said.
It said it does not know how many officers and retired officers will be investigated. The commission can investigate both criminal and misconduct offenses, even after an officer's retirement.
The tragedy occurred when thousands of fans were let through a gate into an already crowded standing area, leading many to be crushed against metal fences and concrete walls.
Horrifying images from the scene showed panicked men, women and children pushed and trampled with nowhere to go as police lost control of the crowd. Of some 25,000 Liverpool fans who had traveled to Sheffield to watch their team play, 96 never came home.