>> yes. >> reporter: prosecutor lisa pellegrini said the defendant thought he was so smart, fooling hisfe and then everyone else by using a poison he assumed was untraceable. she's standing and looking at the jury, points to ferrante, that man right there was one blood test away from the perfect murder. >> reporter: murder, responded the defense? what murder? robert ferrante, it said, did not commit a crime because there was no crime. >> i said as forcefully as i could, we don't believe and we will never believe that autumn klein died from cyanide. >> reporter: the defense was attacking the cornerstone of the commonwealth's case. that blood test with its lethal reading of cyanide could never be trusted, it said. >> there's no way that the result is reliable. >> reporter: throw it out? >> yes. >> reporter: because, he said, the lab initially screwed it up with that 3.35 calculation. it only caught its error months later, correcting the level to 2.2, a still lethal amount of cyanide in autumn's blood. >> it certainly raises its a real issue of its credibility. >> reporter: and gives the d