A Modesto police spokesman says officials suspect the ten-inch knife was planted there just recently.
A man who recently bought the home where Scott Peterson and his wife lived says there's a "red stain on the handle." Gerry Roberts says two of his friends found the knife in a cabinet near the swimming pool in the backyard.
Investigators say the discovery came just days after a tabloid ran a story with rumors that the pregnant woman's throat had been cut.
The police spokesman says investigators have found "no obvious signs of blood or tissue on the knife."
Scott Peterson was sentenced to death in December for murdering his wife and her fetus.