The accused, identified as Sheru, is a labourer and lives around 40 yards away from the girl's residence. He is known to the family, said police.
Sheru was arrested in the evening today but the motive behind the murder could not be ascertained yet. It is also not confirmed as of now whether the girl was sexually assaulted before being murdered, said police.
"Sheru was identified with the help of CCTV footage in which he could be seen riding a bicycle with a sack in it. The sack contained the girl's body, which he dumped at a secluded spot beside the boundary wall of a school in the area," said DCP (Outer) Vikramjit Singh.
Body of the 12-year-old girl, missing since Tuesday evening, was found dumped at a desolated spot near the boundary wall of a school in outer Delhi's Narela area yesterday.
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The girl went missing around 7 pm on Tuesday, following which her siblings informed the police and when she could not be tracked after an intensive search operation, the police registered a case of abduction, police said.
Section of murder was added today in the existing FIR, after which the police arrested Sheru.
The girl's family belong to Bihar. Her parents went to their native village on Sunday, leaving the girl with her siblings, and have not returned yet, police said.
The body has been kept at a mortuary and it would be sent for a postmortem examination as soon as her parents arrive, police added.