Authorities did not give a specific reason for the change. The case of Laci Peterson, 27, of Modesto, was treated as a missing person investigation for more than two months.
"As the investigation has progressed we have increasingly come to believe that Laci Peterson is the victim of a violent crime," said a statement released Wednesday by Craig Grogan, the department's lead homicide investigator.
Grogan said the reclassification has prompted a new $50,000 reward for "information that leads to her location and recovery." Previously a $500,000 reward was offered for information leading to Peterson's safe return home to her family.
Police searched Peterson's home Feb. 18-19 and seized 95 items. Peterson's husband, Scott, 30, was not present.
Police have also searched two of his pickup trucks, seized two family computers and a boat that Peterson bought shortly before his wife disappeared.
Scott Peterson says he last saw his wife the morning of Christmas Eve as he left to go fishing at Berkeley Marina. Police say they don't consider him a suspect but have not ruled him out.
By Jim Wasserman