"F," the fifth person named by John Anthony Walker Jr. in a letter to his alleged Soviet contact, is Walker's half brother Gary Richard Walker, government sources said yesterday.

But the sources said they do not believe that Gary Walker, 24, a third-class petty officer stationed in Norfolk, actively participated in the alleged espionage ring. "F" was listed as an "apparent source of information" in a letter by John Walker to his alleged Soviet contact, according to an FBI affidavit.

According to a second FBI affidavit filed in federal court in Sacramento, Calif., papers found in a search of John Walker's Norfolk house list the first name of "F" as "Gary." The affidavit is currently under seal, but sources confirmed part of its content.

Sources confirmed that the "Gary" identified as "F" is Gary Walker, currently working as an aviation electronics technician assigned to a helicopter antisubmarine squadron.

The sources said it was not clear whether Gary Walker unwittingly gave John Walker information that was passed on to the Soviets.

Lt. Cmdr. Craig Quigley, a Navy spokesman in Norfolk, said Walker did not want to respond to questions.

Sources said no imminent arrests are expected in the espionage case, in which four former and current Navy men have been arrested. But they did not rule out the possibility of additional arrests in the future.

Also yesterday, Sens. William V. Roth Jr. (R-Del.) and Sam Nunn (D-Ga.) said disclosure of the alleged espionage ring underscores the need for cutting the number of security clearances, reducing the amount of classified materials and improving background checks in order to curb an apparent increase in Soviet spying.

As an example of inadequacies in the current system, Nunn said, the military cannot find any record of Michael Walker's security clearance, although officials have said previously he had a "secret" clearance.

Roth, chairman of the Permanent Subcommittee on Investigations, and Nunn, its ranking Democrat, released a list of 22 recommendations for strengthening security, including improving what Roth termed "woefully inadequate" background checks, halving within two years the number of those who hold clearances and setting uniform standards for granting clearances.

In Sacramento, Calif., yesterday, Brenda Leah Reis, the wife of retired Navy radioman Jerry Alfred Whitworth, released a statement declaring her belief in her husband's innocence. Whitworth, 45, was arrested Monday on a charge of conspiring with John Walker to pass sensitive information to the Soviets since 1965.

"During the 12 1/2 years that I have known Jerry, he has never said or done anything which would make me suspect that he would cause harm to the interests of this country," she said. "I believe that he is innocent . . . I will stand by him and support him in this difficult time."

Reis, 30, said that until May 20, the date of John Walker's arrest, she and her husband of nine years "led a quiet life. On that day I first became aware of the allegations against my husband when I was confronted with FBI agents when I arrived home from school."

A doctoral student in nutrition at the University of California at Davis, Reis complained that among the documents seized by FBI agents in two searches of the couple's mobile home were "all of the analyzed data and research necessary for the completion of my dissertation."

Meanwhile, federal prosecutors in Baltimore responded to a request by John Walker's lawyer for sanctions against Assistant FBI Director Bill Baker for making statements about the case in newspaper interviews.

The response conceded that the case has sparked a great deal of publicity, but said that much of the attention has been "generated by the defendant and those arguably acting on his behalf," noting that Walker himself gave interviews to a Norfolk newspaper and that his partner in a Virginia Beach private detective firm, Laurie Robinson, has spoken to reporters.

In West Dennis, Mass., John Walker's ex-wife, Barbara Walker, allowed NBC reporter Bill Schechner to interview her on behalf of a group of reporters gathered outside her apartment.

"I have no intention of making a statement," Barbara Walker said. "I elect not to."

"I will give you this much," she said. "Everything you must know or need to know will be heard in a court of law."