The girl was taken to Broward Health Medical Center, where she was listed in critical but stable condition.
The Broward County Sheriff's office said in a release that the boy found a handgun in the family's Oakland Park, Fla., home while the two had been home alone. The incident occurred just before 7 p.m. on Saturday, Broward County Sheriff's Office spokesperson Dani Moschella said in the release.
Justin Latourrette, a neighbor of the family, told the South Florida Sun Sentinel that he would "never forget" the look on the young girl's face as she was taken out of the house and to the medical center.
"Her shirt was bloody, and you could see through a piece of gauze that she had a hole in her chest above the heart," said Latourrette, who has two young daughters. "Her eyes were wide open in a blank stare, like she was in shock. I'll never forget that look."
The shooting comes less than a week after a 5-year-old boy in Kentucky shot his 2-year-old sister dead with a gun he had received as a gift from his grandmother.