SAN FRANCISCO (AP) _ Two threatening letters sent to newspapers by someone claiming to be the Zodiac serial killer were declared fakes by crime experts familiar with the case.

Many San Francisco Bay area residents remained fearful, however, that Halloween night could bring more bizarre action. There were reports that a symbol used by the killer was left outside the home of the sister of one of the Zodiac's victims.

Police say Zodiac killed at least five people, apparently starting Dec. 20, 1968, with two shootings in Vallejo, 40 miles northeast of San Francisco. The killer, in bizarre, rambling letters to newspapers from 1968-74, claimed he killed at least 37 times since 1966.

The Vallejo Times-Herald and the San Francisco Chronicle received letters this week from someone claiming to be the murderer.

The first letter, which threatened to kill trick-or-treaters on Halloween, was received by the Times-Herald on Wednesday. A handwriting analysis found it to be a phony, police said Thursday.

Vallejo police Capt. Roy Conway, citing handwriting experts from the state Justice Department, said the letter ''was not authored by the authentic Zodiac, but that it is a simulation of another hoax letter written in 1978.''

The 1978 letter, also purportedly from Zodiac, went to the San Francisco Chronicle and also was declared bogus.

The Chronicle also received a Zodiac letter Thursday.

''All I can say is that we've turned it over to the police, who have said that initially it appears to be a fake,'' said City Editor Dan Rosenheim.

Vallejo police said they will take precautions Saturday evening to safeguard residents, but were not specific.

''Whoever is the author of this letter is scaring some people and causing a lot of pain,'' said Conway.

Pamela Huckaby, the 40-year-old sister of 1969 Zodiac victim Darlene Ferrin, reported to Antioch police that she found two pen-drawn symbols of the killer, a circle with a cross through it, drawn on her front and garage door Thursday morning.

She reported on Aug. 15 that a man with a hood approached the house and left a letter, and that a man wielded a knife at her on Aug. 8 and tried to abduct her while she was sitting in her car, Sgt. Robert Low said.

Low said police are trying to determine if the incidents are related to this week's letters.

Robert Greysmith, author of a 1986 best-seller on the Zodiac case, said there was no evidence the Vallejo letter was written by the killer. ''There's nothing original about it. The one thing Zodiac always had was a macabre bent to his writing, and I don't see it here.''

Former police investigator Dave Toschi, who headed the Zodiac investigation for a decade, said Thursday he believes the killer is still alive.