>> reporter: and for a final exclamation point, the defense had a "perry mason" moment up its sleeve. the prosecution had insisted throughout the murder weapon used to bludgeon kathleen peterson was the fireplace blowpoke, only police never found it. almost three months into the trial, one of michael's sons made a stunning discovery in the peterson's basement. >> he immediately runs up to margaret who was at the house and said, "margaret, i think i -- i -- i think i found the blowpoke." >> reporter: the film crew following michael's team for their documentary, "the staircase," was there the day it happened. >> and they look at it. and that's the blowpoke hidden in the corner sort of buried behind something, covered with cobwebs, dirt, bugs, everything, hadn't obviously -- must have been there for years. >> how did they miss that, ron?