"On the results of the forensic psychiatric examination, the woman suffers from chronic mental illness in the form of paranoid schizophrenia," the statement continued.
"A criminal case was sent to the court for a decision on compulsory medical measures."
The diagnosis was made in a notorious Soviet-era psychiatric prison hospital in Kazan where Samsonova was sent earlier this year.
A judge in St Petersburg will now decide on Samonsova's fate. The pensioner will be present at the hearing for which no date has been announced.
It is likely she will be sent for treatment to a psychiatric hospital, rather than face a jail sentence.
Samsonova - who once worked in one of St Petersburg's top hotels which had many foreign visitors - confessed to police that she was responsible for killing Ulanova after poisoning the elderly woman's salad.
She refused to co-operate with investigators on up to 11 more feared killings which she indicated in diary entries written in Russian, English and German.
Police do not rule out further charges but sources admit difficulties in proving other suspected murders because they cannot find human remains.
"We may never know the extent of this granny's killings," said one source.
Among others, she is suspected of butchering are several lodgers, her husband, and her mother in law, who all vanished suddenly and without trace.
A judge ordered her to be sent 950 miles east in a specially guarded train to a high security Kazan psychiatric prison hospital for detailed assessment.
The institution - known for its severe regime - was used by Josef Stalin's vicious secret police chief Lavrentiy Beria to shut away political prisoners during the Soviet era.
It is now called Kazan Psychiatric Hospital of Special Purpose and with Intensive Guarding.
The Samsonova investigation remains 'active' but there are now fears that it may be impossible to find traces of her suspected victims.
One extract of her diary is reported to read: "I killed my tenant Volodya, cut him to pieces in the bathroom with a knife, put the pieces of his body in plastic bags and threw them away in the different parts of Frunzensky district."
Detectives are seeking to match all her alleged chilling diary confessions to known killings in Russia's tsarist-era capital city.
Entries include such mundane phrases penned on unlined yellowing paper as "Slept badly", "Drank coffee", "Take medicines" or "I do not eat".
Blood believed to be from another lodger, named Sergei Potyavin, 44, was reported to have been found in her bathroom, but so far she has not been charged with his murder.
Samsonova admitted to an old school friend Anna Batalina, 67, that she had been under suspicion of killing her mother in law.
Her husband vanished over a decade ago, and she told police at the time that he had met another woman.
Mrs Batalina also claimed she could have ended up as a victim after Samsonova flew into a rage with her, screaming: "I'll kill you. I'll cut you to pieces. I will throw the pieces out for the dogs. Don't make me angry."
For years before her arrest last year she had boasted to friends:"I will be popular and famous."
She told them she would one day cause a 'sensation' without explaining why she believed so.
In court after she was remanded in custody, Samsonova told assembled journalists: "I knew you would come. It's such a disgrace for me, all the city will know."
Later she blew a kiss to the reporters and when judge Roman Chebotaryov invited Samsonova to address the court, she replied: 'It's stuffy here, can I go out?'
She then added: 'I was getting ready for this court action for dozens of years. It was all done deliberately.
'There is no way to live. With this last murder I closed the chapter.'
The judge said: 'I am asked to arrest you. What do you think?'
She replied: "You decide, your honour. After all, I am guilty and I deserve a punishment."
When he announced she would be held in custody, she smiled and clapped her hands.
A police source said despite interrogations and her admission to her elderly friend's murder police cannot make her say where the missing head of her last victim is hidden.
Samsonova was arrested after local dogs sniffed the remains of Mrs Ulanova's limbs which had been dumped in foliage near her block of flats.
In all, Samsonova was caught on camera going in and out of her friend's flat seven times carrying body parts in bags and a saucepan.
Sources say she had deliberately boiled the severed head and hands in an attempt to prevent identification of her victim.
Marina Krivenko, 53, who lived next door to Samsonova, said the Granny Ripper was interested in Soviet serial killer Andrei Chikatilo, executed in 1994.
He was the Soviet Union's worst-ever 'maniac', a bloodthirsty 'vampire' who murdered, sexually assaulted and cannibalised at least 55 in a reign of terror stretching over a dozen years.
This psychopath violated the bodies of his victims, chopping off their tongues, bursting their eardrums, and gouging out their eyes which, he feared, had logged his image.
The neighbour said: "She gathered information about him and how he committed his murders."
Samsonova "was also obsessed with black magic literature," she claimed.
