Dr. Malka Shah, an associate state medical examiner, said 17-year-old Hayley Petit, who was found tied to her bed, had first-, second- and third-degree burns "all over her body," the New Haven (Conn.) Register reported Tuesday.
Her 11-year-old sister, Michaela, also died of smoke inhalation while their mother, Jennifer Hawke-Petit, had been raped and strangled.
On trial for the home invasion is Joshua Komisarjevsky, 31, who could face the death penalty if convicted.
Prosecutors noted Komisarjevsky admitted he tied both girls to their beds and left them there when the fire started, The Hartford (Conn.) Courant reported.
His accomplice, Steven Hayes, was convicted and sentenced to death in a trial last year.
The only member of the family to survive, Dr. William Petit Jr., was not in the courtroom for Shah's testimony Monday. Some jurors were moved to tears, the newspaper said.