Imran Ali is sentenced to death on four counts but Zainab's mother deems this as insufficient for the crimes he committed.
Since Zainab's killer left no clue, police and investigators obtained DNA samples from hundreds of suspects, and Imran's sample matched with her, leading to his arrest.
Local residents said the murder was the 12th such incident over the past year.
Reacting to Imran Ali's death sentence, Zainab's parents Haji Muhammad Amin and Nusrat Bibi voiced satisfaction but they desired to see Imran Ali hanged to death publically.
On January 10, protest broke out in the metropolis against the brutal murder of Zainab after rape and demanded to hang the culprit in public. The victim's father was present for the verdict.
Advocate Mehar Shakeel, counsel for Ali, decided not to plead Imran's case after he confessed his crime.
Her family even claimed that the police refused to take action during the days she was reported missing and her body was found. Relatives also managed to recover CCTV footage of her last movements, according to the BBC.
The verdict was announced at Lahore's Central Jail, reports Dawn news.
Ali was arrested by police on January 23. As of now, Ali has 15 days to challenge the verdict and file an appeal. The prosecutor also said this was "the first case to examine scientific evidence" in a case of this nature.
In Kasur, protesters angered at what they perceived as authorities' inaction, smashed police cars and tried to burn down the police station; two people died in clashes.
Zainab went missing on January 4 in Kasur, Punjab.
The public demand for the immediate arrest of the rapist resulted in an unprecedented search that was joined by nearly all law enforcement and security agencies.
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