A schoolgirl who was feared to have been killed by a flu virus in fact died from deep vein thrombosis, it was revealed on Thursday.
Jodie Mettrick, 15, of Wokingham, Berkshire, died from bilateral pulmonary emboli caused by the blood clots, Reading Coroner's office said.
An inquest into the Holt School pupil's death has been opened and adjourned.
Mystery surrounded her death on 8 February after it was revealed in a letter warning parents of a flu virus.
The coroner has now confirmed that an influenza B viral infection was only a contributory factor and not the main cause of death of the teenager, who died in the Royal Berkshire Hospital.
At the time, the Health Protection Agency sent out a letter to parents telling them of the death and warning them of a flu-like illness affecting schools.
After concerns were raised, the agency later announced that "a communicable disease was not a major contributing factor".
Philippa Nunn, headteacher at The Holt School, said: "The whole school community is shocked and sadden by her death after a short illness.
"Our thoughts are with her family and friends at this sad time."