The FBI began watching John Anthony Walker Jr., accused along with his son of spying for the Soviet Union, about three months ago, a government source said yesterday.

The source said Walker was turned in by his 25-year-old daughter, Cynthia, in addition to his former wife, Barbara Joy Crowley Walker, 47.

Family members of John Walker, 47, a retired Navy communications officer who has been living in Norfolk and working as a private detective, may have suspected Walker's alleged espionage activities, the source said.

In an effort to determine the extent of the alleged activities of Walker and his son, Michael Lance Walker, 22, both charged with espionage, government agents have "conducted lots of interviews and more interviews will be done before this case is through," the source said.

Barbara Walker, who was divorced from John Walker in 1976, declined comment yesterday about her former husband and son when contacted in West Dennis, Mass., where she has been working as a checkout clerk in The Christmas Tree Shops.

"I don't want to talk," she said.

Shortly afterward, she left her job and went to the two-bedroom apartment where she lives with her daughter, Cynthia, and Cynthia's 8-year-old son, Tommy.

Their second-floor apartment is located above a bookstore on the main street of West Dennis, a tourist town on Cape Cod.

Cynthia Walker, one of three daughters of John Walker and the sister of Michael Walker, also declined comment. She works in a dentist's office.

The Walker family includes five brothers, including John and two others with military service, as well as in-laws and grown children in several cities.

John Walker served as a communications officer with "top secret crypto" clearance before retiring from the U.S. Navy in 1976. For the last several years, he has operated three detective companies, one of which specialized in electronic debugging. Walker kept an airplane, a houseboat and a van loaded with electronic equipment.

John Walker's arrest at a Ramada Inn in Rockville came shortly after FBI agents allegedly saw him drop a shopping bag filled with 129 classified documents, including some from the Nimitz, in a wooded area in western Montgomery County.

A Soviet national seen near the alleged "drop site" has left the country, officials said.

Subsequent to Walker's arrest, about 20 FBI agents searched his home at 8524 Old Ocean View Rd. in Norfolk and uncovered material that led to the arrest of his son, Michael, a seaman serving aboard the Nimitz, according to the criminal complaint filed by the FBI in federal court in Baltimore.

Government officials now say that the case has turned out to be far more significant than originally suspected at the time of the first arrest.

Michael Walker, 22, has been serving as a yeoman third class aboard the nuclear aircraft carrier the Nimitz for the last 17 months. Before enlisting in the Navy in December 1982, he lived with his father in Norfolk.

A source familiar with the Walker case said that Barbara and Cynthia Walker contacted the FBI about three months ago. The source said a telephone tip is not sufficient in a case like this.

"They would have to go in and make a written statement . . . and sign it . . . for something this serious," the source said.

The FBI office closest to the West Dennis area is in Hyannis, where the agency has two or three agents assigned to a small office, the source said.

While Barbara and Cynthia Walker kept silent yesterday, the West Dennis townspeople who know them provided some details about the quiet life they have had, despite the recent publicity surrounding the family.

Krista Booker, 22, who works at The Christmas Tree Shops, said yesterday that Barbara Walker had been working there about a year and was "really nice . . . friendly to everybody."

Yesterday, a busy day for the town's tourist trade, The Christmas Tree Shops was jammed with customers buying an assortment of Christmas ornaments, nautical knickknacks and pottery. Barbara Walker, wearing the store's green apron, was one of about six workers manning cash registers. Another half-dozen workers helped bag purchases for customers.

Residents said Barbara Walker moved into the apartment with her daughter in January. They pay about $350 a month for their subsidized housing, according to neighbors.

Tom Bayek Jr., 20, the cook at The Village Deli down the street from the Walkers' apartment, said it was common knowledge in West Dennis that espionage charges had been filed against Barbara Walker's husband.

Bayek said the espionage charges against Michael Walker may have come as a surprise to Barbara Walker. "I think she had to be surprised," Bayek said. "She talked highly of her son in the Navy. She was proud of him."

There has been little or no contact between John Walker and his daughter Cynthia, residents here said. "Tommy's never seen his grandfather -- he says that all the time," Bayek said.

Michael Walker's wife, Rachel, 22, who lives in Virginia Beach, went to Baltimore this weekend in an effort to see her husband. It was not known yesterday whether Rachel made the trip on her own or was summoned by the FBI , or whether she was permitted to talk to her husband.