The public inquiry into the IT failings that led to the wrongful prosecution of hundreds of Post Office branch managers starts hearing evidence from other key witnesses this week.
This new stage of the probe aims to uncover what was behind one of the most widespread miscarriages of justice in UK legal history.
No-one at the Post Office or Fujitsu has yet been held accountable.
The chair of the inquiry said he was "determined to expose the truth".
The BBC says more than 700 Post Office managers were given criminal convictions after a new computer software system, built by the Japanese firm Fujitsu, wrongly showed money was going missing from individual branches between 2000 and 2014.
Thousands of other sub-postmasters lost their businesses, as they were forced to pay back money which was alleged to be missing from their branches.
Testimony
The first phase of the public inquiry earlier this year heard testimony from sub-postmasters who were wrongly bankrupted and imprisoned. They described being treated as criminals by their employers and their communities. Dozens have had their convictions overturned, and many more are in line for compensation.
This next phase of the inquiry will hear from the Post Office, government officials and the Fujitsu. It will examine who knew about the faults in the Horizon software programme, and what they did, or did not do, with that information.
The chair of the inquiry, Sir Wyn Williams, said the evidence he heard through the spring and summer had made a deep impression on him.
In a recorded statement he said he was "determined to expose the truth about these matters".
The process is expected to take several months, with witnesses called who were involved in the design, roll-out, operation and management of the new system.
Earlier this year Jez Thompson, who worked on a team training sub-postmasters to use the new software system, told the BBC that problems were flagged up as Horizon was rolled out.
Glitches
He said glitches and bugs frequently made the software fail to compute the sums, and he regularly passed that information up to his supervisors, who he is sure would have passed it on.
But a member of Fujitsu's UK board, speaking for the first time exclusively to the BBC, said that the board wasn't aware of any significant technical problems when Horizon was being rolled out.
Fujitsu said it has "been co-operating with the current Post Office Horizon IT statutory inquiry since it began and continues to be focused on helping assist its chair and his team".
It says it is committed to providing "the fullest and most transparent information so that key lessons are learned".
But no-one from Fujitsu has apologised, and the company has not offered any financial contribution to towards the compensation bill for the victims of the wrongful prosecutions.
The Post Office has said it is "sincerely sorry" for the impact of the scandal on the wrongly-accused postmasters, and that it believes the inquiry will ensure "lessons are learned".
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