VACAVILLE, Nov. 19 — Convicted mass murderer Charles Manson was transferred from the medical facility here to maximum security Folsom Prison last month after authorities discovered an escape plot involving guns and grenades.

Jess Marquez, program administrator of the unit in which Manson was held at the medical prison, said yesterday the plot was uncovered when authorities found five letters from Manson hidden in another inmate’s cell Sept. 19.

Two letters were addressed to the inmate, who was not identified, and the others were apparently to be smuggled out of the prison to two one time members of the Manson “family,” Lynette Fromme and Sandra Good.

The letters to the inmate “told him to contact these two girls regarding a possible escape attempt,” Marquez said. “He (Manson) wanted to find out about any ‘equipment’ they might have — I deem that to mean guns. He also mentioned grenades.”

Marquez said the letters showed that Manson was “at least starting to think about escape seriously,” although no part of the escape plot was put into action.

Prison officials said they had no idea how the inmate got the letters since he was in another unit with no contact with Manson. The inmate might have been able to smuggle them out of prison, they said.

Medical prison superintendent T. Lawrence Clanon said permission had been given for Manson to carry on a “closely censored” correspondence with the two women.

He said the letters which went through the prison post office contained no hint of an escape attempt. The correspondence was stopped as soon as the illicit letters were found.

Manson, who turned 40 last week, and three other members of his “family” were convicted of murdering actress Sharon Tate and six others in Los Angeles.