NEW DELHI: The immediate trigger for the

brutal murder of a 16-year-old girl in northwest Delhi's Shahbad Dairy on Sundaynight by a man who claimed to be her friend appears to be a fight between the two a day earlier, during which the girl, Sakshi, warned him to stay away.

The attacker,

 Sahil Khan, allegedly saw a tattoo of another boy on her forearm and lost his mind.

Cops have also launched a

 detailed investigation after Khan was found to be wearing a kalava(sacred red thread) in several photographs. Some locals claimed he masqueraded as "Sunny" in the area. There are allegations that the girl wanted to distance herself after knowing his true identity. Special commissioner Dependra Pathak said: "We will conduct a professional investigation and leave no stone unturned to collect all evidence which would enable us to secure

strictest possible punishmentincluding death penalty for the accused."

Sources said while it seemed to be a "crime of passion", there were reasons to probe if the killing was premeditated. Khan had recently purchased a long knife which was used in the murder. The cops are yet to recover it.

The attacker, Sahil Khan, allegedly saw a tattoo of another boy on her forearm and lost his mind

Another CCTV footage shows Khan roaming the street waiting for Sakshi. He is also seen talking to a friend who the cops are questioning.

Sakshi returned from the market and went to a public toilet to change as she had to go for a birthday party. As soon as she came out of there, Khan accosted her in a lane in E block around 8.40pm and attacked her. He kept stabbing her even as dozens of people walked past. In the CCTV footage, a dog can be seen approaching Sakshi as well. A man is also seen trying to stop him briefly but runs away after being threatened by Khan. In the event, no one intervened and Khan ensured the Sakshi was dead in about a minute's time. The police reached after about half an hour and took her to hospital where she was declared "dead on arrival".

Sahil Khan fled to UP after the murder

Khan fled the city after the murder and reached Bulandshahr, his hometown. Using her aunt's phone, he called up his father who was incidentally with the cops. The police then rushed to Bulandshahr and apprehended Khan on Monday afternoon after tracing the cellphone location.

Cops said they did not receive any PCR call about the incident but a local had informed the beat constable around 9:15pm over the phone, after which police officials reached the location.

Sahil's father Sarfaraz had moved to this locality two years ago

On Monday, TOI visited Khan's residence but found it locked. Ramphool Singh, Khan's landlord said, "Sahil's father Sarfaraz had moved to this locality two years ago. He stayed here with his wife, three daughters and his son Khan. They paid Rs 3,500 as rent".

Neighbours said Khan mostly remained quiet and did not interact much. "He would get offended if someone made a religious joke. But he was never into any major fight," said a neighbour. Khan's Instagram account offers a sneak peak into his life. Most of the pictures and videos are of him smoking a "hukkah" with his friends. The cops have questioned his friends who said that he had recently gone to Rishikesh, Haridwar and Vaishno Devi with them.