Julian died earlier this month of internal bleeding, with cuts to his esophagus and gastric irritation, according to the Clark County Coroner's Office. He also had a collapsed lung.
The boy had swallowed a camera battery, the office said.
"Nobody knew, really, what it was," Julian's grandmother, Elena Derbyshire, told the Las Vegas Review-Journal.
Paramedics were called to the boy's house Oct. 18, but even doctors had trouble determining what was wrong.
"They didn't know what it was in the X-ray," Derbyshire said. "They didn't figure out it was a battery until they did the autopsy."
Button batteries, which are found in household items such as cameras, remote controls, watches and hearing aids, react with saliva and create an electrical charge that can burn the esophagus in as little as two hours, according to the National Capital Poison Center. Treatment requires surgery, and permanent damage is common.
"The longer (batteries) are in contact with the tissue, the worse it can be," Nevada Poison Center spokesman Sarah Bruhn said.
The coroner has ruled the boy's death was accidental, but police and the county Department of Family Services are looking into the incident.
"Investigators searched my house. They asked us a bunch of questions," Derbyshire said. "We don't have anything to hide. It's just a tragedy."