Sixteen-year-old Arya K Suresh, who was undergoing treatment at Thrissur Medical College, died following a cardiac arrest at 4pm on 20 July.
Arya's condition deteriorated on Monday morning. The teenager was put on ventilator as she was diagnosed with pneumonia on Saturday.
Along with Arya, two other girls, Athira R Nair (17) and S Raji (16), had gone missing from Konni (Pathanamthitta) since 9 July. Later, the bodies of the duo were found at a railway track near Mankara-Lakkidi area in Palakkad at 7:30am on 13 July.
Forensic reports suggest that the girls died due to the injuries caused after falling off a train.
With Arya's death, the Konni probe has run into a stone wall as investigators were hoping to collect statements from the 16-year-old, who was the only survivor of the accident.