TOKSOVO, Russia -- Volunteers have found 20 sets of bones in a forest near St. Petersburg, evidence of what some believe could be part of a vast burial ground of about 30,000 victims of Soviet dictator Josef Stalin's Great Terror.

So far, the volunteers from the human rights group Memorial have sent nine sets of remains to a forensic laboratory for tests of identifying features including age, sex, cause and time of death.

Memorial will stop digging and declare the site a monument to the victims if the Federal Security Service, the main successor to the Soviet-era KGB, breaks its silence and confirms the area is part of the suspected Stalin-era mass grave, Memorial activist Irina Flige said.

The work is grim in the forest outside Toksovo, about 20 miles northwest of St. Petersburg. With the sound of artillery shells exploding on a nearby army testing range, volunteer diggers stand waist-deep in pits, groping for bones with gloved hands.

Volunteers working with Memorial searched for five years before finding the grave, which they estimate could contain about 30,000 bodies in an area of about 500 acres. They have been digging here since August.

The only other known mass grave in the St. Petersburg area is believed to contain the remains of up to 8,500 people, according to drivers who brought the victims to be execution place in 1937-38, at the height of the Terror.

The drivers were questioned by the KGB in 1965, during a time when Soviet authorities gingerly began to admit the massive scope of Stalin's crimes.

Russian officials have said they believe millions of people died in the communist purges before Soviet leader Josef Stalin's death in 1953.

Yet there was no trace of tens of thousands of other victims who were rounded up in and around Leningrad, as St. Petersburg was known in Soviet times. According to official Soviet-era data, 39,488 people from the region were executed between Aug. 5, 1937, and Nov. 16, 1938. Almost 7,000 people vanished from 1930 to 1936.

Both the KGB and the Federal Security Service kept silent about where the victims were buried, so in the mid-1990s Memorial began publishing appeals for information in newspapers.

The group was founded during perestroika to preserve the memory of the victims of political repression in the Soviet Union. It has since become one of Russia's most respected human rights groups.

Visits by black trucks

Witnesses who had lived in the villages around Toksovo in the 1930s came forward, testifying that black trucks would make nightly visits to the artillery ground. The vehicles stood with their headlights on as shots rang out from the Rzhevsk testing range.

"The range made this area very convenient for NKVD executions," said Miron Muzhdaba, one of Memorial's volunteer diggers, using the acronym for a precursor of the KGB. "They were to conduct the shooting as secretly as possible, so they hoped the testing would somehow hide the fact of real murders."

Muzhdaba pointed to a bone pierced with a neat circle, and said it was the hole from a bullet shot into the nape of the neck -- the classic execution method in Soviet Russia.

"Most of the 20 skulls we've found here over the last month have similar holes in the same part of the neck," he said, adding that the bullet traces mostly matched .45-caliber Colt pistols, the type of gun carried by the Soviet secret police.

Memorial has come across other, indirect evidence that indicates the approximately 30,000 missing victims were buried at the range. It includes official documents and aerial photos showing tire tracks in part of the Rzhevsk range, now overgrown with trees and shrubs.

Anyone buried there could not have been killed by the Nazis because German forces did not reach this area in World War II, Memorial said.

Many of the volunteers lost relatives in the Great Terror and said they were motivated in part by the desire to have a place to mourn.

"I know how important it is for them to have a place where they can come and remember their dear ones," said Anna Reznikova, a graduate student.

Muzhdaba said his great-grandfather was a priest who was executed in 1937. He may buried in the marshy ground where the volunteers are digging, Muzhdaba said.

"When we drive along the road leading to the range, I shiver sometimes," Muzhdaba said. "I can't imagine how it felt for the people in those trucks, realizing that they were on the way to their death."