Mr Pochuyev said Prof Sokolov - who has been receiving hospital treatment for hypothermia - may have been under stress.
Students described Sokolov as both a talented lecturer who could impersonate the French emperor and his generals and a "freak" who called his lover "Josephine" and liked to be addressed as "Sire".
Attorney Aleksandr Pochuyev said on November 9 that his client, Oleg Sokolov, had "signed a plea bargain deal".
Police discovered the two female arms, which had been severed at the elbow, as well as a gun in his bag.
The gruesome corpse and a severed head were found at his nearby apartment on Moyka Embankment, say reports.
Sokolov is a professor at St Petersburg State University and is well known for reenacting moments of Napoleonic history.
One day later, the professor - a man who has been decorated with the Legion of Honor - the highest French order of merit - for his scientific and public works, attempted to dispose of the young woman's body by dismembering it.
The historian, who also taught at Sorbonne University, is the author of books on French emperor Napoleon Bonaparte.
Investigators did not name the particular person however police talking to AFP acknowledged him as Sokolov.
Sokolov was a senior member of the Russian Military-Historical Society headed by Culture Minister Vladimir Medinsky.
Sokolov was also a member of Lyon-based Institute of Social Science, Economics and Politics (ISSEP).
"We learn with horror. the atrocious crime of which Oleg Sokolov is guilty", it said in a statement.
ISSEP was founded by Marine Le Pen's niece, Marion Marechal.