A full public inquiry into failures at Stafford hospital begins its work today.
Health secretary Andrew Lansley ordered the inquiry after agreeing with campaigners Cure The NHS that previous probes backed by Labour ministers had not gone far enough in exposing the failures of management at Mid-Staffordshire NHS Trust.
"We want people to know what happened and why it happened in that hospital," Laura Bailey told politics.co.uk.
"We want to know why it was allowed to slip through the net. Why did the managers get away with it?"
At least 400 more people died at the hospital between 2005 and 2008 than should have done, the Healthcare Commission watchdog found in its initial report of March 2009.
It found patients at Stafford hospital were left for days without food, water or pain relief. Some were even told to lie in their own filth. Others were said to have resorted to drinking out of vases because they were so dehydrated.
A private inquiry led by Robert Francis QC published earlier this year concluded that the Trust's management had been driven by cost-cutting and targets.
But Cure The NHS argued it had not gone far enough and demanded a full public inquiry. "They can't go on oath and not tell the truth," Ms Bailey added.
Mr Francis will preside over the public inquiry which begins today. Senior figures including the NHS' chief executive Sir David Nicholson and former health secretaries Alan Johnson and Andy Burnham are expected to be called.
"The only solution is the greatest openness and transparency," Mr Lansley told campaigners during the general election campaign earlier this year.
"We've got to shed light on these dark tunnels. Unless we have a sense of confidence that the whole culture of the NHS is one of exposing problems, how do we know for sure?"
Mr Burnham is likely to defend his decision to hold a private rather than public inquiry when called to give evidence.
"I didn't commission a full public inquiry for a very clear reason: I believed it was my primary responsibility to help the NHS in Stafford improve quickly," he told politics.co.uk in September.
"The advice I received was a public inquiry would distract the hospital trust and the primary care trust from that objective."
The former Labour leadership candidate, who is now on Ed Miliband's frontbench team as shadow education secretary, added that he understood the "rawness and the sense of anger".
"This was an appalling failure of an NHS organisation for which there are no excuses," he added.
"But I commissioned the exhaustive phase one Francis report which told the story in all of its horror and did that so the NHS would learn the lessons."
The Stafford hospital proved a huge issue at the 2010 general election campaign, when minister David Kidney lost his seat to the Conservatives' Jeremy Lefroy with a 7.4% swing.
The inquiry will deliver its final report by March 2011.