The document, known as the Costello Report, was never disclosed to the lawyers, jury or judge at the original trial of five men who were jailed for life over the 2002 gangland killing in Pattingham.
In 2012, the convictions of the five men – Adam Joof from Willenhall, Antonio Christie, from Great Bridge and Levi Walker, from Birmingham, as well as Michael Osbourne and Owen Crooks, both from Wolverhampton – were quashed after details of the failings were exposed by a whistleblower.