The US police assigned the human remains found in Florida on Wednesday to the fiancé of Gabby Petito, who was killed on a road trip.
A comparison with dental records showed that it was the remains of Brian Laundrie, the FBI said on Thursday evening on Twitter.
The case had kept the US in suspense for weeks and also attracted international media interest.
The FBI announced on Wednesday that human remains and personal belongings of Laundrie had been found in a nature reserve.
The 23-year-old lived in the nearby town of North Port in southwest Florida.
According to the family, he had stayed in the nature reserve more often.
Refused to testify
Laundri's fiancée Petitio had disappeared while traveling across the United States with her fiancé.
Her body was finally discovered after a feverish search in mid-September in Grand Teton National Park in the state of Wyoming in the western United States.
She was strangled, according to forensic medicine.
Laundrie had initially returned alone from the trip to Florida in early September, where he lived with Petito.
When Petito's parents reported their daughter missing, the 23-year-old initially refused to testify and eventually disappeared.
He was subsequently wanted by an arrest warrant - not directly for Petito's death, but for credit card fraud after the death of his fiancée.
The case also caused so much attention because the couple shared their experiences during the adventure originally planned for four months on the online services Instagram and Youtube.
The photos showed a smiling couple and impressive landscapes.