The boy, Nikhil (name changed), 14, had been eyeing the device that belonged to his friend Sagar Dolui, who went to the same school with him. "He (Nikhil) wanted to listen to songs but Shiuli, Sagar's sister, refused to part with it," said Susmita Chakraborty, a neighbour of the Doluis.
Nikhil asked Shiuli to bring the MP4 player with her and accompany him to the Tilpara barrage, near Suri town, about 230km from Calcutta.
The boy told her he would take pictures using the camera in the device, the police said. He took Shiuli on a bicycle, and as they approached the barrage, asked for the MP4 player.
When she refused, he pushed her off the bridge after snatching the device from her, according to the police. She fell 30ft below, her head hitting the rocks on the side of the barrage.
Swapan Banerjee, the inspector-in-charge of Suri police station, said: "The boy then went down to check if she was alive. When he found her unconscious but breathing, he strangled her and dragged the body behind a bush."
It was "a murder with intention", the police officer said. "He murdered her in anger because she refused to give him the gadget. We have arrested him." The MP4 player has not been found yet. MP4 player
is a marketing term that was created in the People's Republic of China for portable media players that comply with certain standards and formats.
In Calcutta, psychiatrist Ranadip Ghosh Roy said the gadget might not have been the only reason behind the murder. "The boy needs counselling."
About a fortnight ago, Shiuli's father Swapan had bought the music player it played songs and videos and also had a camera for Rs 2,000. Shiuli, who had a speech problem, grew fond of the device and would often listen to music on it.
Relatives said Swapan, who paints buildings, bought the MP4 player, though too costly for his means, so his children would have some entertainment at home. "There is no TV or music system in their house. So, he thought this would provide some entertainment to the children," said Rakhi Dolui, Swapan's niece.
One newspaper reported the boy remained remorseless and that he was crazy about such gadgets.