A teenager was detained on Thursday for his alleged involvement in the death of a 17-year-old girl whose body was stuffed in a gunny bag that was found in a canal near Regent Park last week.

Police said the teenager had admitted that he had helped the prime suspect, whose name was found tattooed on the girl's hand, commit the crime.

The cops have launched a search for the prime suspect.

"According to the detained teenager, his friend, who was close to the girl, had masterminded the killing. He said they did not realise that the girl was alive when they stashed her inside the gunny bag and threw the bag into a canal," said an investigating officer.

A senior Kolkata Police officer said the girl left her home, on the city's southern fringes, on July 21. She went to a place where she met her friend and they stayed there overnight. The detained teenager and another girl were also present there. The other girl left the next day.

"She was killed in a taxi. The detained teenager and the alleged killer took the body to a factory to dispose it of. The detained boy has said the other person hatched the murder plan because of a strain in his relationship with the victim. We can verify the statement only after he is caught," said the officer.

The two accused allegedly stuffed the girl's body in the gunny bag, apparently thinking that she had died, and threw it into the canal on July 22. The gunny bag was found floating near Regent Park on July 23.

The post-mortem revealed the girl was alive when she was thrown into the water and that her death was by drowning.