The girl went missing from her home in Shanti Mohalla of Gandhi Nagar on May 18 and a complaint was lodged in this connection by her brother Vijender Kumar at Krishna Nagar police station.
Avinash (19), who was in contact with the girl, was arrested from Old Delhi railway station yesterday, said Rishi Pal, DCP(east).
During interrogation, he disclosed that he along with his cousin who is a juvenile, murdered the girl and put her body in a plastic bag before disposing it of in the Najafgarh drain.
The cousin and the girlfriend (both juveniles) were apprehended by the police today.
Search for the body of the girl is on, police said.