MOSCOW - Russian investigators arrested a nanny and charged her with murdering a young child in her care on Monday after local media broadcast footage of a woman brandishing a severed infant's head near a busy Moscow metro station.Moscow investigators said in a statement the unnamed nanny was from Central Asia and that she was undergoing psychiatric testing to see if she was mentally sound and understood the significance of the crime they said she had committed.Lifenews.ru, an online news portal with close contacts to the police, published video footage of a woman it identified as the nanny. Clad in black, she was seen wandering around the street holding up a child's severed head high in the air.The portal said a policeman had approached the woman to check her documents and that she had responded by removing the child's head from a bag and begun shouting that she had killed the infant.One reporter, from the RBC daily, said she had heard the woman screaming "Allahu Akbar" (God is Great)."I was on my way to the metro station from home," Polina Nikolskaya, the reporter, told Reuters."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."In other footage of the scene, the woman can be heard shouting about the end of the world while proclaiming herself a terrorist.Investigators said the murdered child was three or four years old and that the nanny had killed the child in the family's Moscow flat before setting fire to the premises and fleeing.The motive for the crime was unknown, they said.