Pfc. Emmanuel Mensah initially escaped the deadly fire but reentered the burning building at least three times to rescue other residents and family members, New York Police Department detectives reported.
When firefighters later entered the building, they discovered Mensah's remains. Based on his body's location, emergency responders determined Mensah died looking for more people to rescue.
The fire started in the kitchen of a first-floor apartment as a 3-year-old boy was playing with the burners, FDNY Commissioner Daniel Nigro said. It's the worst fire tragedy in New York City in at least a quarter of a century. The victims killed were seven adults and five children -- ranging in age from 1 to 63.
WATCH: Coverage of the Bronx fatal fire
Mensah joined the New York Army National Guard in December 2016 and recently returned home to the Bronx from military training in Virginia.
He was slated to begin serving as a wheeled vehicle mechanic this month with the National Guard's 107th Military Police Company at Fort Hamilton in Brooklyn.
His family is originally from Ghana, and Mensah was a permanent legal resident.
Private Emmanuel Mensah was a first generation immigrant, a soldier, and a New Yorker. He gave his life rescuing his neighbors in the Bronx fire. His heroism exemplifies the best of our city. Rest in peace.— Bill de Blasio (@NYCMayor) December 30, 2017
Staff Sgt. Ruben Martinez-Ortiz, who recruited Mensah, said he was an excellent soldier.
"I knew from the moment I met him his heart was as big as our National Guard family," Martinez-Ortiz said. "He was ready to serve our nation and community. Pfc. Mensah was the embodiment of what our Army Values stand for."
PHOTOS from the scene of the Bronx fatal fire: