SAN DIEGO, Jan. 13 — Lynette Fromme, sentenced to life in prison for trying to kill President Ford, will not be prosecuted for sending a threatening letter to the head of the Environmental Protection Agency, the U.S. Attorney said today.

“We have reviewed the letter and the contents and, after extensive research, we find that there is virtually no chance of a successful prosecution,” said Terry J. Knoepp.

He said the law requires an “unambiguous threat of personal injury” directed by the sender to the recipient to sustain a successful prosecution and that “the Fromme letter does not fall within the case law requirements.”

The government said Sunday that Miss Fromme sent a death threat letter from the federal Metropolitan Correctional Center in San Diego, where she is being held, to Russell Train.

The Manson Family member mailed the letter to Train from the jail, in a sealed Department of Justice envelope that she addressed with postage paid by the government. Officials said it reached the EPA chief in Washington Jan 2.

Miss Fromme, who became acting head of the “Family” when Charles Manson went to prison, adopted the environment as a cause for the group, which allegedly had a “death list” of corporate executives marked for retribution by a “People’s Court” for causing pollution.

The letter addressed to Train as “you weak mealy mouth,” said in part:

“You are warned now from the international people’s court who can be as mean as you are weak and that is beyond mutilation.

“As head of the EPA for the entire country you are warned and will have your own life and face shoved in the pollution which you allow to go on and on and on. The state of the country is clearly an example of your uselessness.”

Authorities said Miss Fromme’s name and prison number were on the letter along with initials IPR under a swastika, in the style of the Manson cult Miss Fromme, convicted of poking a .45 automatic at President Ford in Sacramento, is being held in the jail pending transfer to a federal prison.

Jail warden J.D. Williams said he will revoke her right to send privileged mail. Under Justice Department policy, prison authorities cannot inspect such mail between inmates and lawyers or government officials.