All day long, FBI counterintelligence agent Bruce Brahe II had been kicking wet trash along roads that John Walker had driven, but it wasn't until he found a "crumply, dry" grocery bag beside a telephone pole in rural Montgomery County that he thought he might have found something.

When he opened the bag and found "clean trash" inside, like a rinsed out soft drink bottle with the cap on it, "I got excited . . . it was a classic type of Soviet dead drop . . . when I smell one, I know one," Brahe said during testimony today in the federal court trial here of Walker's brother, Arthur Walker.

The discovery of that bag of trash -- and secret Navy documents underneath -- set off the arrest on espionage charges of John Walker in Rockville hours later and the arrests in the following days of his brother, retired Navy Lt. Cmdr. Arthur Walker; son Michael, a Navy seaman; and friend Jerry Whitworth, a retired Navy radioman. Authorities said it is one of the most significant spy rings in decades.

From the testimony of FBI agents here in support of the charges against the defendants in the Walker case comes the most detailed description to date of the activity leading up to the arrest of John A. Walker.

The FBI had been investigating John Walker for six months because they had been tipped off by his ex-wife, Barbara, about his alleged espionage.

But in May, John Walker said over his telephone, wiretapped by the FBI, that on the weekend of May 19 he was going to perform a job in Charlotte, N.C., that "only he could do," according to an FBI affidavit.

The FBI assumed he was going to contact his Soviet "handler" and lined up dozens of agents to tail him. But leaving his Norfolk home that afternoon in his silver and blue van, John Walker did not drive south toward North Carolina, instead heading north to Richmond, on to I-95, ultimately across the Cabin John Bridge, and into Montgomery County, FBI agents said.

The FBI agents followed him in cars and at least one airplane, as he pulled over, started again, made U-turns, and performed numerous other driving tricks to "dry clean" himself. The term "dry clean" refers to attempts to lose a pursuer or discover who might be following, the agents explained.

The FBI agents said that they lost him from the air at 5:50 p.m. when he was driving in heavy forest in Montgomery County, but that they spotted him again at 7:48 p.m.

FBI agent Francis McEnzie Jr., watching from a plane, testified that he saw Walker stop his van many times, get out, walk around, then drive away.

"On several occasions, he'd stop apparently to see who was behind him, the headlights, or whether vehicles altered speed," McEnzie said.

Authorities said Walker apparently thought he had lost anyone tailing him earlier in the evening. Meanwhile, based on observations from the air, Brahe's FBI search team, trained in spotting "dead drops," had been tromping through wet grass at "literally scores" of roadside areas where Walker had stopped that day, Brahe said. "We didn't know precisely what we were looking for," he said.

The FBI search teams had to be careful because, since Walker was driving aimlessly, he might drive by again as they were looking, Brahe said. Around 9 p.m., Brahe and colleagues arrived at a tree on deserted Partnership Road in Poolesville that FBI spotters said Walker had driven by four times.

"We made the conclusion the tree was a reference point," Brahe said. Thirty feet away was a telephone pole with a "no hunting" sign on it.

"In every way, it looked like a Soviet drop site," he testified. Then he found the dry grocery bag.

Opening it, he found the tidy trash, and a pile of papers underneath, wrapped in white plastic and taped. "Almost plebian, pre-World War II trade craft," Brahe said.

He ran to a nearby cornfield to inspect the bundle out of sight. Then four FBI agents returned to the area near the pole to wait for a Soviet agent to arrive.

In the next two hours they watched Walker -- or someone driving his van -- drive by and search the "drop" area three times, presumably to make sure the Soviet had picked up the package, Brahe said. Each time, the FBI agents were within 10 feet of him in the dark, Brahe said.

The agents averted their faces as Walker approached because faces can be seen in the dark, Brahe said. For that reason, none of the agents could positively identify John Walker as the man who searched the area, but authorities said they had no doubt it was he.

A Soviet diplomat spotted by FBI agents nearby that night hurriedly left the country a few days later, authorities said. Walker was arrested early the next morning in a Rockville motel.

Arthur Walker's lawyer, Samuel Meekins, has objected repeatedly to the FBI testimony about John Walker's arrest at his client's trial, saying that Arthur Walker was not with his brother or in Poolesville that evening. Meekins also said that what happened in May is irrelevant to charges that Arthur Walker passed confidential documents to his brother in 1981 and 1982.

Assistant U.S. Attorney Tommy Miller said he was producing FBI testimony about the surveillance of John Walker because it is evidence of a conspiracy to commit espionage, one of seven counts with which Arthur Walker has been charged.

U.S. District Judge J. Calvitt Clarke Jr. has upheld Miller's argument that Arthur Walker still could have been in a conspiracy three years after last allegedly passing documents, because he did nothing to end the supposed espionage ring.

Also in court today, a Navy intelligence officer specializing in Soviet naval affairs testified that one of the confidential documents Arthur Walker is alleged to have passed, a training manual on repairing damage aboard the USS Blue Ridge, is "a bible for sabotage" because it "identifies specific points of vulnerability on the ship."

The "damage control book" about the USS Blue Ridge, a command ship that carries top officers of the Seventh Fleet in the Pacific, could tell an enemy what type of explosives to use to sink the ship, said Capt. Edward Sheafer, a senior intelligence officer with the Navy's Atlantic command in Norfolk.

Meekins said the documents did not contain sensitive information and "didn't put anybody's life in jeopardy."