FBI agents patiently shadowed accused Soviet spy John Anthony Walker Jr. for six months, waiting to arrest him until the first time they saw him drop off classified documents, FBI Assistant Director Bill Baker said yesterday.

In another development, government sources said Navy Secretary John F. Lehman Jr. is considering having Walker and his brother, Arthur James Walker, recalled from retirement to active Navy duty so they can be court-martialed.

Navy officials want to assure that both men would be severely punished if convicted, and they are concerned that federal prosecutors might agree to a plea bargain with either man in order to uncover evidence about the extent of information allegedly passed to the Soviets, the sources said.

Baker said in an interview that although confidential sources had gone to the FBI six months ago with information allegedly implicating John Walker, this material alone was not enough to arrest him.

"We were looking for overt acts," he said. "It took his long trip from Norfolk to here, the process he went through to clean himself [avoid surveillance] and the act of putting" a bag full of documents at the alleged "drop site" for the FBI to be able to make its case.

"We patiently watched him for six months and waited for him to commit espionage," Baker said.

John Walker was arrested May 20, the day after FBI agents, according to an affidavit, saw him drop a bag filled with classified Navy documents at a remote site in Montgomery County. Soviet vice-consul Aleksey G. Tkachenko, who left the country four days later, had been spotted near the alleged drop site, according to court papers and testimony.

Baker said the FBI believes that John Walker was the linchpin of the alleged conspiracy to pass defense secrets to the Soviet Union, and that it was "a possibility" that the two other Walker family members who have been arrested -- his brother, retired lieutenant commander Arthur Walker, and John Walker's son, Navy Seaman Michael Lance Walker -- each may not have known that the other was involved.

"We don't see any letters saying, 'Tell Uncle Art . . . ,' " Baker said.

"We're not sure at this time we even have a family loop," Baker said. "What we have at this time is John Walker dealing with Arthur Walker and John Walker dealing with Michael Walker."

John Walker, 47, and Michael Walker, 22, were indicted Tuesday on six counts of espionage. Arthur Walker, 50, an engineer at VSE Corp., a Chesapeake, Va., defense contractor, was arrested Wednesday night on espionage charges.

Baker said that additional arrests are expected in the case and that agents are sifting through the "wealth of information" gathered through the arrests and various searches. He said that there are "a few suspects" in the Norfolk area, where all of the Walkers live.

Sources said that Navy Secretary Lehman met with his top lawyers yesterday and asked them to assess the advisability of recalling John and Arthur Walker to active duty so they can be court-martialed.

John Walker, a communications specialist with the Navy, retired in 1976 after a 21-year career. Arthur Walker, who served on submarines and as an antisubmarine warfare instructor, served 20 years before his retirement in 1973.

The move may also be attractive to the Navy because of the possibility of limiting the disclosure of confidential information or details that might compromise Navy operations.

Pentagon General Counsel Chapman B. Cox said last night that "under some circumstances" it would be legal to recall retired Navy people to active duty for military prosecution. But he said he had not looked into the details of whether this would apply to the Walker case, adding that "this matter would be worked out between Justice and the Navy."

Assistant U.S. Attorney Michael Schatzow said after a bond hearing for Michael Walker Wednesday that under a Justice Department agreement with the Navy, the Justice Department would proceed with trying both Arthur and Michael Walker before the Navy brought any proceedings against them. Arthur Walker had not yet been arrested when Schatzow made those comments.

Government agents in the continuing investigation have been questioning persons in California, according to sources. John Walker was assigned to the Crypto Repair School at the U.S. Naval Schools Command in Vallejo in 1963 and served as assistant director of a radio school at the Naval Training Center in San Diego from 1969 to 1971.

Baker said that agents are convinced that the motivation for the alleged spying was financial. "Money is the driving reason why," he said.

But he said that agents have not located additional caches of money or valuables beyond those found in searches and already publicly revealed. "You're not talking tremendous amounts of cash changing hands," Baker said. "We've found ironically in these [espionage] cases that while money is the reason, the amount of money requested has been pitifully low."

According to FBI affidavits filed in federal court in Norfolk and Baltimore, John Walker gave his son $1,000 last year in exchange for documents he had passed earlier; Arthur Walker admitted receiving about $12,000 from his brother in return for material he had furnished, and a confidential source reported seeing John Walker retrieve a paper bag containing $35,000 from a roadside in the Washington area about 15 years ago.

Searches of John Walker's house, safe deposit box and other possessions turned up ten 100-ounce silver bars, according to the affidavits. Walker owns a 1977 camper, a single-engine airplane and a houseboat. On a financial form he filled out in order to obtain a court-appointed lawyer, he listed his assets as his house, valued at $70,000, and property in Norfolk and South Carolina worth a total of $120,000. He said he owes $45,000 on the house and properties.

Baker said that the FBI had no clue about the alleged spy ring until its confidential sources came forward about six months ago. "It's what we call a walk-in," he said. He said that the confidential sources implicated only John Walker.

Sources have said that Walker's ex-wife, Barbara Joy Crowley Walker, and one of his three daughters turned him in to the FBI. Chester Buck, the Hyannis, Mass., landlord of Cynthia Walker, one of the Walkers' daughters, said he arranged a meeting with the FBI for Barbara Walker several months ago.