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 told the newspaper.
Ingesting batteries -- especially small, disc-shaped "button batteries" found in watches and small electronics -- has become more common as the batteries themselves become more common, according to a report from the Centers for Disease Control and Prevention. The agency identified 14 battery-swallowing deaths in the United States between 1995 and 2010; all of them involved children under age 4.
Batteries sometimes pass through the body, but they can also pose a choking hazard or cause burns by producing a small electrical current or leaking acid.
"We always refer them to go to doctor and get an X-ray," said Sarah Bruhn, spokeswoman for the Nevada Poison Center. "Batteries are never good to be swallowing."
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."
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