Federal prosecutors in the case of accused spy Jerry Alfred Whitworth have renewed their effort to use letters Whitworth allegedly wrote to convicted spy John Anthony Walker Jr. at Whitworth's trial.

A person identifying himself as "RUS" sent four letters to the FBI's San Francisco office between May and August 1984, first offering to expose a "significant espionage" system in which he had participated, and later withdrawing the offer.

Prosecutors contend that Whitworth, 46, a retired Navy communications expert charged with passing the Soviets sensitive information about military codes, wrote the letters. However, they have been unable to link him to them through fingerprints, comparisons of typewriters or watermarks on the paper and envelopes.

In a 43-page memorandum filed in federal court in San Francisco Thursday, Assistant U.S. Attorneys William S. Farmer and Leida B. Schoggen argued that the jury should decide whether Whitworth was the author of the RUS letters.

"The proof at trial will show that there are so many identifiable similarities between the RUS letters and writings and knowledge of defendant Jerry Alfred Whitworth that no reasonable juror could doubt his authorship," the memo said.

The facts stated by RUS "correspond perfectly" to those known by Whitworth; such things as the postmark, the type style, and the paper used are "consistent" with the theory that Whitworth wrote the letters, and "the writing style, the layout and the irregularities of language and punctuation all match those of Jerry Whitworth," it said.

U.S. District Judge John P. Vukasin has refused to admit the letters into evidence at Whitworth's trial on espionage charges, tentatively scheduled to start next week. At a hearing in December, Vukasin termed the letters "in essence confessions," and said if the government could prove Whitworth wrote them, there would be virtually "no need for a trial."

Among the stylistic similarities cited in the memorandum are the use of rhetorical questions, the phrase "and etc.," the spelling of "though" as "tho," the use of slash marks in place of the word "and," and "unusually frequent" use of parentheses.

The memo also provided a preview of some of Walker's expected testimony against Whitworth, his former Navy colleague.

It said Walker would testify that when he originally recruited Whitworth in 1974, he did not reveal that he was dealing directly with the Soviets, but rather "hinted to Whitworth that organized criminal groups dealt in military secrets just like they dealt in drugs and other forms of contraband."

Eventually, the memo said, Whitworth "clearly became aware that Walker was dealing with the Soviets and . . . expresssed various forms of concern about this fact."

Walker and Whitworth then agreed in the event that Walker was arrested, their "cover story" would be that they were dealing with a private intelligence organization -- "precisely what John Walker said to FBI agents after his arrest on May 20," according to the memo.