The foster parents have been arrested from their residence in Rajiv Awas Vihar here following postmortem report which confirmed the couple's alleged involvement in the girl's death, superintendent of police O P Tripathi said.
"There are indications of sexual assault and the girl being beaten to death," Tripathi said.
"It was yet to be confirmed by the medical experts the kind of sexual assault or torture the girl was inflicted to by the couple," he said.
A case under sections 302 (murder), 323 (voluntarily causing hurt), 376, 377 and 120B of IPC was registered against the couple, SP said.
The accused would be produced before a court later in the day, he said.
The accused, Rajesh Sengar and Bebi Sengar, had adopted the girl child from their relative in Uttar Pradesh four months ago.
On the night of September 26, Rajesh was spotted by the neighbours while he was taking her body wrapped in a plastic mat to dispose it, he said.
The neighbours called the police who took the body and sent it to MY Hospital for postmortem.
During interrogation, Rajesh told the police that the girl had complained of chocked throat, for which he was taking her to a private hospital, the SP said.
Postmortem report confirmed that the girl died of head injury, beaten by a belt kind of thing, and suffocation in air-pipe (throat), he said.
Meanwhile, enraged over the incident, residents of Rajiv Awas Vihar ransacked the couple's house this morning. They staged a road blockade and demanded strict punishment for the girl's foster parents.
Police force was sent to the area to maintain law and order.