Teton County Coroner Dr. Brent Blue announced that Petito's cause of death was ruled as strangulation. "We hereby find the cause and manner of death to be the cause of death by strangulation."
It's estimated that she died about three to four weeks before her body was found.
The body of Petito, 22, was found on Sept. 19 near an undeveloped camping area in remote northern Wyoming along the border of Grand Teton National Park. Blue previously classified Petito's death as a homicide -- meaning her death was caused by another person -- but did not disclose how she was killed pending further autopsy results.
Petito had been on a cross-country trip with her boyfriend. She was reported missing Sept. 11 by her parents after she did not respond to calls and texts for several days while the couple visited parks in the West.
Her boyfriend, Brian Laundrie, is considered a person of interest in her disappearance and remains unaccounted for.
Laundrie's father joined in the search for his son on Thursday.
Petito's case has led to renewed calls for people to pay greater attention to cases involving missing Indigenous women and other people of color, with some commentators describing the intense coverage of her disappearance as "missing white woman syndrome."
The search for Laundrie has generated a frenzy, with TV personalities like Duane Chapman -- known as Dog the Bounty Hunter -- and longtime "America's Most Wanted" host John Walsh working to track him down.
Petito and Laundrie posted online about their trip in a white Ford Transit van converted into a camper. They got into a physical altercation Aug. 12 in Moab, Utah, that led to a police stop, which ended with police deciding to separate the quarreling couple for the night. No charges were filed, and no serious injuries were reported.
Investigators have searched for Laundrie in Florida and also searched his parents' home in North Port, about 35 miles (56 kilometers) south of Sarasota.
Federal officials in Wyoming last month charged Laundrie with unauthorized use of a debit card, alleging he used a Capital One Bank card and someone's personal identification number to make unauthorized withdrawals or charges worth more than $1,000 during the period in which Petito went missing. They did not say to whom the card belonged.
The Associated Press contributed to this report.