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A criminal investigation was launched after the discovery of the remains of a child aged three to four, whose body bore traces of a violent death, in a charred flat on Narodnoye Opolcheniye Street in north-western Moscow, it added.
"According to preliminary information, the child's nanny, a citizen of one of the Central Asian states born in 1977, waited for the parents and elder child to leave the flat and then, for reasons not established, murdered the infant, set fire to the flat and left the scene," it added in the statement.
Police sources told Russian news agencies that the victim had been decapitated and the head removed from the scene by the nanny.
'I am your death'
The suspect has been named by Russian media as Gyulchekhra Bobokulova, from the former Soviet republic of Uzbekistan.
Russia's tabloid news website Life News, which specialises in crime coverage, reports that the victim was a little girl.
According to Life News, a policeman asked to see her identity documents outside Oktyabrskoye Polye metro station, at which point she pulled the head of the child out of a bag and began screaming that she would blow herself up.
http://www.bbc.com/news/world-europe-35685981No traces of explosives were found on the suspect after her arrest, the site adds.
Amateur video posted online shows a black-clad woman, apparently the suspect, outside a metro station shouting the Islamic phrase "Allahu Akbar" (God is Great).
"I am a terrorist," she cries. "I am your death."
Russian journalist Polina Nikolskaya witnessed the incident. She told Reuters: "I was on my way to the metro station from home. She was standing near the metro entrance and caught my attention because she was screaming Allahu Akbar.
"I saw that she had a bloodied head in her arms but I thought it was not real. People in the crowd said it was real."
Other witnesses says the woman walked up and down outside the metro station for some 20 minutes, shouting and brandishing the head.