VACAVILLE, Jan. 17 — Authorities dismissed assault charges Wednesday against a state prison inmate who allegedly splashed convicted mass killer Charles Manson with paint thinner and set him ablaze, saying the prisoner apparently acted in self-defense.

Manson, 48, serving a life sentence for the 1969 slayings of actress Sharon Tate and eight others, suffered second- and third-degree burns in the Sept. 25 attack. He was returned to his maximum-security cell following treatment in the prison infirmary.

James Highsmith, deputy district attorney of Solano County, said the charges were dropped because it appeared likely that inmate Jan Holmstrom, 36, acted in self-defense when the incident occurred.

He said it appeared probable that Holmstrom acted in self defense, in view of Manson’s prison record, in which attacks on other inmates have been reported.

“There is a long history of prison violence and threats by Manson, to show that he is a violent character, apart from the Tate-LaBianca killings,” Highsmith said.

Holmstrom, a member of the Hare Krishna sect, was described by prison authorities as a “psychiatric case in remission” at the time of the attack. He has since been transferred to a special segregation union for disciplinary reasons, said Lt. George Galaza, a prison spokesman.

Prison officials said Manson was in the hobby shop of the institution, located 50 miles northeast of San Francisco, when the other inmate drenched him with paint thinner and tossed a match on him.

Manson was burned over 18 percent of his body, mainly on his face, scalp and hands, in the morning attack.

Manson, whose prosecution and murders were detailed in the book “Helter Skelter” by former Los Angeles prosecutor Vincent Bugliosi, had threatened Holmstrom because of his religious beliefs, Holmstrom told prison officials.

Manson, who was originally sentenced to death, was given a life term after the death penalty was ruled unconstitutional by the U.S. Supreme Court. He was also convicted of the murders of Gary Hinman, a musician, and Shorty Shea, a worker at an isolated movie ranch that housed Manson’s followers.

Although Manson did not commit the murders himself, there was testimony at his trial that he ordered his followers to kill.

Three of Manson’s female followers, Patricia Krenwinkle, Susan Atkins and Leslie Van Houten, are imprisoned in Southern California serving life terms. Another follower, Charles “Tex” Watson, is serving a life sentence at the California Men’s Colony at San Luis Obispo.