Samaria Rice says gun fake, hear scary 911 call, the boy’s mother, told a local TV station that her son had gone to a recreation centre across the street from their home in Cleveland with friends and his older sister.
Police officers said they were investigating the circumstances of the shooting.
They said they were responding to an emergency call, which described a male brandishing a weapon.
According to a recording, the caller expressed doubts that the gun was real.
“There’s a guy in here with a pistol,” he said. “It’s probably fake but he’s like pointing it at everybody.”
When police arrived, a group of people were sitting around a table under a gazebo next to a car park, according to an account given to Northeast Ohio Media Group by Jeff Follmer, Cleveland Police Patrolmen’s Association president.
He said the responding officers were not made aware the caller had described the gun “as probably fake”.
“Besides we have to assume every gun is real,” he said. “When we don’t, that’s the day we don’t go home.”
Ed Tomba, deputy Cleveland police chief, said the weapon lacked the orange safety indicator usually displayed on the muzzle of guns.
The boy did not make any verbal threats toward the officer or point the gun, but reached into his waistband and grabbed it after being told to raise his hands, said Mr Tomba.
“That’s when the officer fired,” he said.
The two officers involved in the shooting have been placed on administrative leave during the investigation.
The death comes as the US awaits the outcome of a grand jury investigation into the police shooting of an unarmed black teenager in Ferguson, Missouri.
Riots erupted in August after Michael Brown, 18, was shot dead in broad daylight by a white policeman. Fresh trouble is expected if the grand jury decides there is insufficient evidence to prosecute the officer, Darren Wilson.