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."
The caller added: "I don't know if it's real or not".
Jeff Follmer, Cleveland Police Patrolmen's Association president, 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.
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 Ed Tomba, the deputy Cleveland police chief.
"That's when the officer fired," he said.
The two officers involved in the shooting have been placed on administrative leave during the investigation.
- Telegraph Group Ltd, AP