The girl was playing and watching TV on Thursday afternoon with the 4-year-old boy and his 5-year-old sister when she found the gun, police Sgt. Michael Woody said. He said she picked up the weapon, pointed it at the boy and shot him once in the chest.
The boy was taken to a hospital, where he was pronounced dead, Woody said. A man who lives at the home on the city's west side was the only adult there at the time and is believed to a grandfather, Woody said. It wasn't immediately known whether the gun belonged to him.
An investigation is ongoing and the Wayne County prosecutor's office will determine whether charges should be brought, he said.
"It's a tragic incident," Woody told The Associated Press. "It's really not anything more than that. There was no malicious intent. There was nothing there to suggest that the grandfather put those children in harm's way, but the investigation will continue."
The boy's name wasn't immediately released.
Woody said the shooting reinforces the need for those with guns to keep them secure - especially when there are children around.
"This is a very senseless ... incident that could have been avoided," he said.
On Thursday night, investigators went in and out of the brick house. Police cars blocked traffic. Neighbors told the Detroit Free Press that they didn't hear a gunshot.
"I was just devastated," said 77-year-old neighbor Dorothy Jameson. "I was wondering why two 4-year-olds have access to a gun in the first place."
Dennis Queensbury, who told The Detroit News that he has a concealed pistol license, said gun owners need to secure their firearms.
"I have three children: 16, 9 years old and 20," Queensbury said, after watching officers at the scene. "But I have a gun lock. You have to have a lock and safes, and be careful."