A white Ohio police officer was justified in shooting dead a black 12-year-old boy holding a pellet gun moments after pulling up beside him, two independent reports have said.

Both a retired FBI agent and a Denver prosecutor found the rookie Cleveland patrolman who shot Tamir Rice exercised a reasonable use of force because he had reason to perceive the boy - described in an emergency call as a man waving and pointing a gun - as a serious threat.

The killing of Tamir has become part of a national outcry about minorities, especially black boys and men, dying during encounters with police.

The reports were released by the Cuyahoga County Prosecutor's Office, which asked for the outside reviews as it presents evidence to a grand jury that will ultimately determine whether Timothy Loehmann will be charged over the death of Tamir last November.

"We are not reaching any conclusions from these reports," Prosecutor Timothy McGinty said. "The gathering of evidence continues and the grand jury will evaluate it all."

He said the reports, which included a technical reconstruction by the Ohio State Highway Patrol, were released in the interests of being "as public and transparent as possible."

But Subodh Chandra, a lawyer for the Rice family, said the reports showed the prosecutor's office was avoiding accountability.