CHENNAI: Police on Sunday arrested a 22-year-old engineer for the murder of doctor and postgraduate student S Sathya for her cellphone, which he sold for Rs 1,200 after stabbing her to death in her flat in Kilpauk.

Investigators said they identified Harintham Depnath as the killer after tracking down the Micromax phone by its IMEI number to a man who purchased the handset from a shop in Maduravoyal. They found that the engineer had sold the phone to the Maduravoyal shop owner.

The Maduravoyal shop owner told police he had taken a photograph of Depnath because he was behaving shiftily. Investigators used the picture to catch the killer and discovered that he stayed in the same apartment complex as the doctor.

S Sathya, 32, a first-year MS (obstetrics and gynaecology) student at Kilpauk Medical College, shared her flat with a senior from the college. The mother of two was from Perambalur and had come to the city two months ago.

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Police said Depnath had completed his BTech in May and was looking for a job. He stayed in a flat on the first floor of the apartment with his brother, a doctor at a multispecialty hospital on Greams Road.

"Sathya had fallen asleep after returning home on Thursday after a night shift," an investigating officer said. "Harintham admitted that he stealthily entered the house by unlatching the door from outside. As he picked up the cellphone, the doctor awoke and raised an alarm."

Depnath panicked and assaulted Sathya, then used a pillow to muffle her cries for help.

"When she started to suffocate and swooned, he took a knife from her kitchen and stabbed her repeatedly in the neck," the officer said. "He left the knife in her neck, went home and washed his bloodstained shirt."

"He then sold the cellphone to a mobile phone dealer in Maduravoyal," he said. "A customer named Gopal purchased the phone and activated it on Sunday. We questioned the shop owner and he gave us the picture of Depnath."

Depnath told police he kept tabs on Sathya's schedule and knew she would be sleeping after a night shift. "He decided to commit the crime before Sathya's flatmate Sangeetha returned home in the afternoon," the officer said.