A prominent Russian history professor has confessed to murdering his young lover and former student and dismembering her body in a gruesome crime that has sent shockwaves across Russia.

Oleg Sokolov, a 63-year-old expert on Napoleon Bonaparte who received France’s Legion of Honour from Jacques Chirac in 2003, was arrested on Saturday on suspicion of murder after he was hauled out of the icy Moika River with a backpack containing a woman’s arms.

“He has admitted his guilt,” Sokolov’s lawyer Alexander Pochuev told AFP, adding he regretted what he had done and was cooperating. The historian was being treated for hypothermia in a hospital on Sunday.

Sokolov was reportedly drunk and fell into the Moika, a tributary of the Neva, in central St Petersburg as he tried to dispose of body parts near the offices of investigators.

After disposing of the corpse, he reportedly planned to commit suicide at the Peter and Paul Fortress, one of the former imperial capital’s most famous landmarks, dressed as Bonaparte.

Russian police investigators as they conduct searches on the Moika in Saint Petersburg on Sunday. Photograph: Olga Maltseva/AFP via Getty Images

Sokolov, who teaches history at St Petersburg State University, President Vladimir Putin’s alma mater, told investigators that he shot and killed his lover during an argument and then sawed off her head, arms and legs, media reported.

Pochuev suggested Sokolov may have been under stress or emotionally disturbed.

Police discovered the decapitated body of Anastasia Yeshchenko, 24, with whom Sokolov had co-authored a number of works, and a blood-stained saw at his home.

The historian, who also taught at the Sorbonne in Paris, is the author of books on French emperor Napoleon Bonaparte. He acted as a consultant on several films and took part in historical re-enactments of Napoleonic wars.

He and his lover both studied French history and liked to wear period costumes, with Sokolov dressing up as Bonaparte.

Students described Sokolov as a talented lecturer who could impersonate the French emperor and his generals.

Many expressed dismay, saying that Sokolov had long been known for his hostile behaviour.

Media said that Sokolov also beat up and threatened to burn with a hot iron and kill another female student in 2008 but was never charged.

“What happened is simply monstrous,” a lecturer at the university told AFP. Speaking on condition of anonymity, he said Sokolov was dedicated to his work but was also emotionally unstable and abused alcohol.

Some people who have known the lecturer for years said they were flabbergasted and at a loss to explain the tragedy. “I cannot get my head around it,” historian Ilya Kudryashov said.

His former student, Fyodor Danilov, said Sokolov was regarded as one of the university’s best lecturers but also an eccentric man who at times yelled in French.

A spokeswoman for the Grand Chancery of the Legion of Honour, speaking to AFP, indicated that Sokolov might be stripped of his award. The final decision rests with the French president, Emmanuel Macron.

Sokolov was a senior member of the Russian Military Historical Society headed by the culture minister, Vladimir Medinsky. The organisation immediately sought to distance itself from the controversy.

Sokolov was also a member of Lyon-based Institute of Social Science, Economics and Politics (ISSEP), which announced on Saturday that he had been stripped of his position on its scientific committee.

ISSEP was founded by Marion Maréchal, the niece of Marine Le Pen, leader of the far-right National Rally party.