Two large buttons on either shoulder of Zainab's alleged murderer's jacket were a significant clue that helped the police zero in on him, BBC Urdu said in a report.

According to BBC Urdu, an officer on the case shared the details of how the police went through the investigation process with them.

The Kasur district has a population of over 3 million people, of which 0.7 million live in Kasur city. Even after filtering out women, children and old men from that demographic, the police were still left with at least 60 to 70 thousand men — only one of which was their suspect, BBC Urdu said.

According to BBC Urdu, out of these thousands, the search was further narrowed down to 1100 men, all living within the radius of the investigation.

The Pakistan Forensic Science Agency collected DNA samples of all of these men and compared each with that of Zainab's rapist, as their 1,110 strong DNA database gradually exhausted.

It was during this tedious process that Imran Ali's DNA was found to a perfect match with that of Zainab's rapist.

Once that evidence came to light, police went looking for Imran — now the prime suspect — at his house and were later able to arrest him with the assistance of his mother, BBC Urdu said.

During the raid, police found a jacket with large buttons on both shoulders, similar to the one the rapist was seen wearing in the CCTV footage that surfaced when Zainab first went missing.

The police officer told BBC news that this was a tricky piece of the puzzle as in the black and white CCTV footage the jacket appeared to be white, which meant it could be of any dark colour in actuality.

According to the officer, finding the jacket among Imran's belongings helped them make sure that he was indeed the man they were looking for.

According to BBC Urdu, Imran's mother and paternal uncle said they did become suspicious when they first saw the CCTV footage on TV and thought the man in the video looked like Imran but they did not bother to tell the police about it.

Imran, per the report, said in his confession that he killed the girls due to the fear of being caught for raping them. Of his eight victims, he said he raped and murdered five in under-construction houses, while three others — including Zainab — were raped and murdered close to the heaps of trash where their bodies were found.

Imran also said that he killed the children and disposed of their bodies the same day as raping them, BBC News said.