SAN FRANCISCO, Jan. 18 — A member of the Charles Manson family was arrested Monday at her home in Petaluma on a charge of sending two threatening letters through the mail.

Patricia Maureen Gillum, also known as Misty Hay, was taken into custody without incident, Special FBI Agent-in-Charge Charles Bates said.

An affidavit filed by an FBI agent with the complaint said one letter was sent to the president of the Sierra Club in San Francisco, Paul Swatek, and another to A. A. Emmerson, an Arcata businessman.

The first letter — addressed to “Sierra Club President” and signed “Misty” — threatened to injure and kill the conservation club leader and others, the FBI agent said.

It read in part: “Charles Manson has fake breakers watching you people now…do your part in stopping the cutting down of trees, etc., or else you’ll be chopped up yourselves. For every tree limb you let be cut down, you shall have one of your limbs chopped off!! Take heed of this mean letter or die.”

The letter to Emmerson, signed simply “M,” included the threat: “Stop chopping, or you will get yours, one way or another…work for a change…or die.”

The affidavit said FBI laboratory analysis determined the letters were prepared by Miss Gillum and a latent fingerprint of hers was found on the envelope sent to the Sierra Club president.

The state Department of Corrections, the affidavit continued, said Misty Hay is a member of the Manson family and is Miss Gillum.