MOSCOW — Russian police today arrested a woman who was seen waving the severed head of a small child outside a Moscow subway station. She is suspected of killing the child when it was in her care, officials said.

Videos posted on Russian news websites show a woman dressed all in black, holding the severed head and shouting “I am a terrorist” in Russian, although most of what she says is incomprehensible. In some of the videos the woman is tackled by men who appear to be police.

The Investigative Committee released a statement saying a woman was arrested today on suspicion of killing a child aged 3 or 4 in an apartment near the metro station in northwestern Moscow and then setting the apartment on fire. The woman, a 38-year-old native of one of the former Soviet republics in Central Asia, was believed to be the nanny and appeared to be mentally unstable, the statement said.

The official report said the child’s body found in the apartment showed signs of a violent death. Russian news reports said the child had been decapitated.

Investigators said they were trying to determine the suspect’s motive.