Police rescued the girl, identified as Lyuba, in Moscow on Sunday after neighbors heard the child crying and smelled a foul odor coming out of the residence, The Associated Press reported.
Anna Kuznetsova, a children's rights official in Russia, said Lyuba does not know how to talk, and smells her food before she eats it.
"It's very disturbing to watch. She hasn't been taught to sleep on a bed, it was very hard for us to put her to sleep. She's also afraid of water," she told Russian news agencies, the AP reported.
Footage by Russia's Investigative Committee, Russia's equivalent to the FBI, shows the apartment inundated in plastic bags, bottles and other trash and what seems to be dirt smeared on the walls. The committee did not give any further details about Lyuba's condition.
Investigators said that the water and electricity were not working because Lybua's 47-year-old mother had not paid the bills, according to the AP.
Authorities arrested the mother, and are investigating her for conspiracy to murder, the AP reported. The mother told investigators that she left her child unattended only for "short periods" when she worked.
The investigative committee will also probe the response of police officers and social workers. Emergency services rescued the girl a few days after the first police squad refused to force its way into the apartment, according to the AP.