Nora Quoirin’s body was found beside a stream about 1.6 miles (2.5km) from the jungle resort of Dusun on Tuesday.
Malaysian Police said there was no suspicion of abduction or foul play.
The 15-year-old’s body was discovered following a 10-day search after she disappeared on 4 August.
The teenager, whose family have strong connections to Derry, died two or three days before she was found, police believe.
Nora was born with holoprosencephaly, a disorder which affects brain development, and had been described by her family as vulnerable.
Her parents had previously said they didn’t believe she would have wandered off alone and suspected she had been abducted.
Negeri Sembilan state police chief Mohamad Mat Yusop, said: “For the time being, there is no element of abduction or kidnapping.”
He said: “The cause of death was upper gastrointestinal bleeding due to duodenal ulcer, complicated with perforation.”
Further analysis would be carried out on samples taken from her body, he said, adding that Nora’s family was free to take her home.
They said Nora, who lived in London, had “truly touched the world” after her disappearance sparked a huge search operation in Malaysia and good wishes from across the globe.