Handwriting experts at the state crime lab eased the minds of Bay-Area parents by finding that a letter purportedly from the Zodiac serial killer is, in fact, "a simulation of a hoax letter written in 1978," acting Vallejo Police Chief Roy Conway said. The letter, sent to a Vallejo newspaper, threatened to harm children on Halloween. Police attribute six random slayings to Zodiac, but the killer wrote bizarre coded messages to newspapers in which he claimed to have murdered 37 people between 1966 and 1974. Zodiac, described as a pudgy, bespectacled man about six feet tall, has not been heard from in 13 years. Both the San Francisco and Vallejo police departments still consider the case open. Kati Corsault of the state Department of Justice said the latest hoax letter still is being examined for fingerprints and its author could be prosecuted for sending a threatening letter.