The parents of the minor were in Saudi Arabia to perform umrah when the incident occurred. Police also confirmed that she was strangled to death after being raped.
Her body was found in a rubbish dump on Tuesday, police said.
The community has been furious at the authorities for what they see as a failure to investigate such cases.
The murder has caused outrage in Pakistan, and mass protests broke out in Kasur, located in Punjab province, on Wednesday. According to officials, two protesters were killed in the riots that erupted in the aftermath of Zainab's brutal rape and murder.
The injured have been shifted to the hospital for medical assistance whereas the security personnel are busy in dispersing the locals.
Punjab Chief Minister Shehbaz Sharif visited the victim's family Thursday morning assuring them the culprits will be caught and punished, local media reported.
Lawyers boycotted judicial proceedings at district bar in protest and demanded early action into the matter.
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The inspector-general of police has been asked to submit a report on Zainab's murder to the court in Punjab.
Police say DNA recovered from Zainab's body suggests the perpetrator might be a family acquaintance.
CCTV footage purportedly showed Ansari, 8, being abducted last week outside her home in Kasur district.
In a different kind of protest, Kiran Naz, a TV anchor, brought her daughter to a news programme and blamed the government's inefficiency, calling the incident a murder of humanity.
Punjab government in the evening said six personnel, including four policemen and two civil defence personnel, who allegedly opened fire at the protesters have been arrested.
The incident was widely reported by local media, and people across the country took to social media to condemn the crime.
This is the 12th similar case to occur in just a two kilometre radius in the past year.
According to data compiled by NGO Sahil previous year, about 11 cases of child sexual abuse are reported from across the country everyday.
He tweeted: "The condemnable & horrific rape & murder of little Zainab exposes once again how vulnerable our children are in our society". This is not the first time such horrific acts have happened. "Those societies that can not protect its children are eternally condemned.Just presided over a meeting of police and administration officials".