The Class X student found in the school washroom with her wrist slashed and a polythene bag wrapped around her head had died of asphyxia, police said on Saturday, quoting the preliminary post-mortem report.

The polythene bag cut off the oxygen supply and is believed to have caused the death of Krittika Paul, 14, of GD Birla Centre for Education. At least 26 slash marks were found on her left arm, the police said, adding these were self-inflicted.

“She died of asphyxia. There was a knot mark around her neck, from tightly tying the polybag that covered her face,” said an officer at Lalbazar.

Several pencil sharpener blades were found inside the washroom where she was sitting with her back to the door, bolted from inside. The police have ruled out foul play.

“Plastic bag asphyxia is a known cause of suffocation,” said B. Suresh Kumar Shetty, professor and head of forensic medicine at the Kasturba Medical College, Mangalore. “With a plastic bag on the head, the oxygen level decreases and the carbon dioxide level in the body increases.”

“When carbon dioxide rises beyond a point, the person goes into a state of near unconsciousness where the reflexes drop and removing the bag will be difficult even if he or she wants to,” Shetty said.