The shooter was the boy's 4-year-old cousin.
Sgt. Michael Woody, a Detroit police spokesman, said the children were playing in the back bedroom of a Detroit home when the cousin found a rifle — loaded and not locked in a case — underneath the bed.
He said the girl pointed the gun at her cousin and pulled the trigger, shooting him in the chest.
Woody said police were called to the home at about 4:50 p.m. He said the child was taken to Henry Ford Hospital, where he was pronounced dead on arrival.
Police said there was one adult, the grandfather, in the home at the time of the shooting.
Friday, police said a man is in custody on a charge of being a felon in possession of a weapon, and a woman is in custody on an outstanding warrant.
It is unclear whether charges will be brought as a result of the shooting.
Police Capt. Rodney Cox called the shooting a "tragic, tragic accident," but one that was avoidable. Cox said the department stands behind responsible gun ownership, but "preliminarily, this does not appear to be responsible gun ownership."
Woody said guns should be kept unloaded, locked in a case and out of the reach of children.
"This is an example of how we must maintain gun safety," Cox said.
On Thursday night, crime scene investigators went in and out of the tidy brick house, passing by an illuminated porch light. Police cars blocked traffic on the street. The block is a mix of occupied and run-down, vacant homes.
Neighbors who live nearby told the Detroit Free Press that they didn't hear any gunshots and aren't well-acquainted with the people who live at the house.
"It's so sad," said Jennifer Robinson, 29, of Detroit, as she left the home of a relative who lives on the street. She looked toward the flashing red and blue lights of a police car and shook her head.
"I was just devastated," said another neighbor, 77-year-old Dorothy Jameson. "I was wondering why two 4-year-olds have access to a gun in the first place."
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