Preview: American Girl, Italian Murder

The CBS News broadcast "48 Hours Mystery" has been following the Amanda Knox case from the beginning and in a special report at 10 p.m. Eastern and Pacific time Saturday Correspondent Peter Van Sant examines a key piece of evidence used to link the killing to Knox and her former boyfriend Raffaele Sollecito.

In court, the prosecution presented two crucial pieces of DNA evidence to the jury, starting with the knife allegedly used to kill Meredith Kercher.

Is it the murder weapon? The prosecutor says that it was found at Sollecito's apartment.

There was no blood on the knife. But investigators say they did find DNA - Knox's on the handle and Kercher's on the blade.

It sounds like the murder weapon. She handles it, she stabs and it leaves DNA on there.

"That's not necessarily true cause there's always a transfer of evidence when you live with people," CBS News "48 Hours" Consultant Paul Ciolino said. "Amanda's DNA is on a knife that she used that was on a kitchen drawer at Raffaele's apartment. That knife was never at the murder scene. The DNA that purportedly belongs to Meredith on that knife was tested out of existence by the Italians. It was never verified."

There's another problem with the knife. Police discovered two faint knife impressions on Meredith's bed. When the alleged murder knife is compared to those outlines, it doesn't match.

"That's the knife they want you to believe is the murder weapon, but it's not the murder weapon," Ciolino said. "It doesn't fit the outline on the sheets."