It was the second incident in the United States in recent days involving young children accidentally shooting others.
In Toms River, New Jersey, the two boys were playing in the yard of the 4-year-old's home Monday evening. The younger child left the yard, went inside his family's house, retrieved a .22-caliber rifle and returned to the yard, according to Sgt. Robert Rankin of the Toms River police department.
While the parents of the 4-year-old were in the yard nearby, the boy emerged with the rifle and a shot rang out, Toms River Police Chief Michael Mastrondardy said during a news conference Tuesday morning.
"It's really difficult to deal with," Daniel Watkins, uncle of the 6-year-old boy, told CNN affiliate WCBS. He said the two children were friends. "This is something that shouldn't have happened. It's horrible."
The Ocean County prosecutors office is investigating the incident to determine whether charges will be filed.
On Saturday in Tennessee, the 48-year-old wife of a deputy sheriff died after a 4-year-old boy shot her, authorities said.
The deputy sheriff was showing a relative some of his guns at the deputy's home when his wife and the child came into the room. The child manged to pick up a loaded handgun and fired a single shot, killing the woman, authorities said.