An independent review of police custody deaths in England and Wales has been announced by the home secretary.
The review, which will also cover serious non-fatal incidents, comes after a string of high-profile cases and allegations of wrongdoing.
Theresa May said she had been struck by the “pain and suffering” of families amid seeming evasiveness and obstruction.
The review will examine “procedures and processes” in such situations.
The plan to review how deaths in custody occur – and how they are investigated – comes after the police complaints watchdog was attacked for inadequately getting to the bottom of a number of fatalities.
In 2013, a review found that the Independent Police Complaints Commission had committed a series of blunders in its investigation of the 2008 death of Sean Rigg, a mentally ill man detained at Brixton police station.
The original investigation concluded police had acted reasonably and proportionately – a finding rejected by a jury at Mr Rigg's subsequent inquest.
Mr Rigg's sister Marcia Rigg-Samuel told the BBC the review had been “a long time coming”.
She added: “What I want, and I speak for myself and on behalf of other families, is that this review is effective and brings real change on the issue of deaths in custody, and how families feel and how we are treated, and that there's proper accountability.”
Ms Rigg-Samuel also said families should be “at the core of the review”, to give them confidence that changes will be made.
“When you lose a loved one in state custody, it's bad enough having to deal with the death. What's extraordinary is the systematic failures, and the answers that we cannot get, from the state officials. It's devastating for any family.”
In 2010, allegations that the IPCC had failed to properly investigate the death of another mentally ill London man, Olaseni Lewis, led to the High Court quashing the watchdog's original findings.
The review is expected to cover the lead-up to deaths, the immediate aftermath and how families are helped or supported during official investigations.
It will assess whether police officers properly understand mental health issues, the availability of appropriate healthcare, the use of restraint techniques, and suicides in the first 48 hours of detention.
Mrs May said: “Police custody is the place where a number of dynamics meet. It is the place where dangerous and difficult criminals are rightly locked-up, where officers and staff regularly face violent, threatening and abusive behaviour, and where the police use some of their most sensitive and coercive powers.
“But it is also a place where all too often vulnerable people, often those with mental health problems, are taken because there is no other place to go.”
The home secretary pledged the review would have the experiences of families at the heart of its approach – and its chairman, yet to be appointed, would be someone prepared to ask “difficult questions”.
She said: “I have been struck by the pain and suffering of families still looking for answers, who have encountered not compassion and redress from the authorities but what they feel is evasiveness and obstruction.
“I have also heard first hand the frustration of police officers and staff, whose mission it is to help people but whose training and procedures can end up causing bureaucracy and delay.
“No-one – least of all police officers – wants such incidents to happen, and I know everyone involved takes steps to avoid them.
“But when such incidents do occur, every single one represents a failure – and has the potential to undermine dramatically the relationship between the public and the police.”
Deborah Coles, of the charity Inquest, which provides advice to people bereaved by a death in custody, said it was “too early to tell” if the review was more of a public relations exercise, or a real attempt to bring about effective systemic change and accountability of police officers.
“For the review to be effective bereaved families, their lawyers and Inquest will need to play an integral role in the review, and the reviewer will need to take full account of their views and experiences.
“It must also address why so many previous recommendations from reviews, inquiries and inquests have not been acted upon.”