Police released a sketch of the man, who was seen outside Ansari's home in Kasur, in northeast Pakistan, on January 4, according to the Joint Investigation Team's office in Lahore.
Malik Ahmad Khan, a Punjab government spokesman said there had been eight attacks on girls in the past two years that were now linked by DNA evidence -- and that Zainab may be the ninth victim of a suspected serial attacker. Authorities are awaiting DNA results to confirm the link, he said.
All nine victims were raped, and eight of them were killed, Khan said. The lone surviving victim remains in the hospital, he said. The bodies of the other eight were all found in the city of Kasur.
Zainab's rape and murder has prompted outrage across Pakistan and families in Kasur say they are fearful to let their daughters go out.
Khan added that the authorities were testing the DNA of around 50 people who were currently in custody but didn't explain why these individuals had been singled out for testing.
Previously, officials said Zainab was one of twelve young girls that had been raped -- 11 of them murdered -- in the past twelve months in the Pakistani city located near the Indian border but believe this is no longer the case.
Father: 'Allah made her so lovely'
The young girl's family told CNN that they are having trouble coming to terms with her death, mourning the loss of the daughter who loved mangoes and ice cream, and who wanted to be a doctor or a teacher.
"Allah had made her so lovely that anyone who met her instantly fell in love with her," her father, Muhammad Amin Ansari said.
After his young sister's brutalized corpse was found in a trash pile about 100 meters from the family home, her face caked with mud, her brother said that he couldn't sleep.
"She was so funny," Abu Zar Ansari said. "I miss her laughter, I can't forget her laughter."
Neighbors say that the city is wracked with fear as the specter of a predator looms large.
"We are not letting our girls get out. We're terrified about their safety after what happened to Zainab," neighbor Muhammad Iqbal said.
Zainab was taken from near her home in Kasur on January 4 while her parents were in Saudi Arabia on a pilgrimage. Her uncle reported her missing the next day, police said.
An autopsy confirms Zainab was strangled and sodomized, the medical officer who carried out the examination, Quratulain Atique, said last week.
There were torture marks on her face and Zainab's tongue was crushed between her teeth, the doctor said.
Zainab's rape and murder have shocked the country, with widespread demonstrations accusing authorities in Punjab province of doing too little to protect children.
Over the weekend her father echoed sentiments that are stirring across the country, including the Punjabi capital of Lahore, where rallies were also held. Elsewhere in Pakistan, people took to social media to voice their anger over Zainab's death.
Following a spate of killings of young girls over the past two years, Ansari said that residents in Kasur had been existing in a state of dread.
"For the last two years, we are living in fear. Parents are scared to send their kids outside," Zainab's father, Muhammad, told reporters last week.
He told CNN that he held the authorities at least partially responsible.
"I am concerned about this system, that my daughter could have been saved," Ansari said.
"The acts of the authorities, their disinterest and lack of help has led to the death of my child."
Friday's demonstrations in Kasur remained peaceful. The authorities deployed troops in the town after earlier protests turned violent.
The provincial government on Thursday removed the head of the unit that was investigating the deadly assault on Zainab, replacing him with another police officer.
In roughly the same time frame that the girls were killed, three boys were also raped and murdered in Kasur district, Khan said.
Those cases are not connected to each other or to the suspected serial killer, he said.
Kasur district was previously at the center of a child sexual abuse scandal
in which a gang of up to 25 men was accused of blackmailing scores of children into making sex videos between 2009 and 2014. The abuse was uncovered in 2015, and multiple suspects were arrested.