>> reporter: what was jim knapp doing with those documents? and something else, perhaps very significant. >> there was blood on the doorknob of the door that led from the main house into the backyard garage. >> reporter: the blood became evidence item #805 collected days after carol's death. and whose dna was mixed with carol's blood? jim knapp's dna. >> just like the thumbprint, the question becomes when did jim knapp's dna get put on that door handle? >> reporter: robertson clearly had his suspicions. and steve's sister sharon did too. >> so you've felt all along that jim knapp should have been a suspect and wasn't. >> he should've been investigated. there were a lot of red flags that were concerning. >> reporter: knapp told sheriff's deputies he was no where near carol's house when the murder happened.