Modesto police say they feared Scott Peterson was preparing to flee to Mexico when they arrested him on Friday.

Peterson was arrested by plainclothes police in San Diego before noon. The arrest was made hours before DNA results were revealed showing that his wife and unborn son were the bodies that washed up on the shores of the San Francisco Bay this week.

Attorney General Bill Lockyer says Modesto police and state agents used wire taps on phones and radio transponders on vehicles to keep track of Peterson. He says that Peterson indicated moments before his arrest that he discovered he was under police surveillance.

Modesto Police Chief Roy Wasden says authorities obtained an arrest warrant yesterday. Wasden wouldn't discuss possible motives for the alleged murder, but said he believed the slaying took place in Modesto.

Peterson is expected to be arraigned early next week on a capital murder charge.