ST PETERSBURG -- Sixty years after the momentous murder of Leningrad Communist Party boss Sergei Kirov, historians are still arguing over who was behind what one leading academician calls the "Crime of the Century."


Amid the mist of claims, counter-claims and conspiracy theories which swirl around this turning point in history, just two facts are indisputable.


At 4:37 P.M. on Dec. 1, 1934, as Kirov was walking down a murky corridor towards his office in Leningrad's Smolny building, he was shot in the back of the head by 30-year-old Leonid Nikolayev.


That same night Joseph Stalin launched the Great Terror of the late 1930s, when countless millions of people were shot and imprisoned.


Some historians are convinced that the highly paranoid Stalin, alarmed by Kirov's growing popularity within the party, decided to have him killed before he could become a potential rival for power.


One such expert is Dr. Robert Conquest, a Stalinologist who has taken a close interest in Kirov, who was 48 when he died.


"Nothing I've seen has changed my view that Stalin was directly responsible for Kirov's death," he said. He added that there is overwhelming circumstantial evidence Stalin was to blame.


"On one level it was word of mouth only," said Conquest in a telephone interview from Stanford University in California. "But a court of law is not the same as a court of history." Stalin's smoldering suspicions of the charismatic Kirov seem to have been confirmed in early 1934 during the 17th Communist Party Congress, when elections to the policy-making Central Committee showed Kirov's star shone brightest of all.


Contemporary accounts relate that of the 1,966 delegates, only a handful voted against Kirov, compared to almost 300 against Stalin. Worse for Stalin, there were tentative suggestions he should be replaced by Kirov.


Of the delegates present, 1,108 were shot over the next five years before the 18th Party Congress in 1939.


Many of those executed have been posthumously pardoned and the State Commission for the Rehabilitation of the Repressed is plowing through the countless documents on the Kirov case.


"There are no new conclusions to be drawn. Unfortunately nothing has come up. I think Stalin was too cunning to leave any direct traces," said commission official Yury Shigachev, in charge of the Kirov file.


Conquest believes Stalin made it known to the dreaded NKVD secret police that he wanted Kirov dead.


Leningrad NKVD officers, looking through their files for possible assassins, came across Nikolayev, who had a string of grievances against Kirov after being expelled from the party and sacked from his job in April 1934.


But this version is dismissed by retired Communist Party historian Anna Kirilina, who has spent 30 years studying the Kirov case.


She says she was given access to material which no one else has seen, including the original investigation into Nikolayev carried out immediately after the assassination.


"I personally am convinced that Nikolayev carried out the killing alone," she said in an interview. "Conquest did not work in the archives. I did."


"Nikolayev was a very vengeful, up-tight person, who saw injustices everywhere. He thought Kirov personified evil," said Kirilina, who -- unlike Conquest -- believes Nikolayev's wife had at one stage had an affair with Kirov.