MODESTO, Calif. – Hundreds of police and volunteers have searched parks and rural areas for a pregnant woman who, according to her husband, vanished on Christmas Eve after saying she was going to walk the dog.

Officials used dogs, horses, helicopters and rafts in a huge Christmas Day search for 27-year-old Laci Peterson, who is eight months pregnant. A neighbor found her golden retriever, its leash still attached, on Christmas Eve.

Investigators have also scoured parks and abandoned buildings, and questioned registered sex offenders and paroled convicts.

Scott Peterson told police that he last saw his wife on the morning of Dec. 24 as she prepared to walk their dog in a park. He said he was leaving for a daylong fishing trip. When he returned home and found his wife missing, he called police, authorities said.

A bloodhound that was brought in to help with the search did not head to the park, but instead led investigators to a boulevard, indicating the woman may have gotten into a car.

The bloodhound also made a beeline toward the San Joaquin River, and crews inspected a section of the river Monday. But they did not find anything.

Detectives said Scott Peterson gave investigators a receipt Monday from the marina where he said he went fishing. But Detective Doug Ridenour said: “At this point, we are not ruling him or anybody out.”

A reward for Peterson’s return has grown to $500,000.

Modesto is a small city in the California’s Central Valley farming region.