Investigators have found no hard evidence a Cleveland police officer who fatally shot a 12-year-old black boy carrying a pellet gun ordered him to raise his hands before opening fire.

Documents released on Saturday by the prosecutor handling the racially charged case detail the moments before the brief, deadly encounter - and how the responding officers seemed almost shell-shocked as Tamir Rice lay dying outside a recreation centre.

Cleveland police have said the officer who fired the fatal shot, Timothy Loehmann, told Tamir three times to put his hands up, then opened fire when the boy reached for the pellet gun tucked in his waistband.

Grainy, choppy surveillance video shows Loehmann firing two shots within two seconds of his police cruiser skidding to a stop near the boy.

Cuyahoga County sheriff's detectives investigating the shooting wrote that, based on witness interviews, it was unclear if Loehmann shouted anything to Tamir from inside the cruiser before opening fire.

The report comes two days after a judge ruled that the two police officers involved should face criminal charges over the death of Tamir in November.

A series of fatal incidents, including Tamir’s case, involving black men and boys stirred racial tension and resulted in widespread protests last year.

Interests of transparency

Prosecutor Tim McGinty has said the case, as with all police-involved shootings, will be taken to a grand jury to determine whether criminal charges should be filed against Loehmann or his partner, Frank Garmback. McGinty said he decided to release the investigative file now in the interests of transparency.

"If we wait years for all litigation to be completed before the citizens are allowed to know what actually happened, we will have squandered our best opportunity to institute needed changes in use of force policy, police training and leadership," McGinty said.

"These cases involve peace officers/public employees whose decision to take a fellow citizen's life must be evaluated to determine, by law, whether the police officer's action was reasonable under the circumstances and therefore justifiable."

The report details an FBI special agent's memory of his interaction with Loehmann, whom the agent described as "distraught".

"He seemed like a guy that was put in a very difficult situation and had to make a very quick decision based upon what he believed was an imminent fear of death or serious physical injury to himself and reacted to it," he told investigators.

Loehmann, 26, and Garmback, 47, have been criticised for not giving Tamir first aid.

Loehmann's attorney, Henry Hilow, said he has not had a chance to read the investigative file and said the officer committed no wrongdoing.

"The events were a tragedy, but there was no crime committed," he said.

On Thursday, Cleveland Municipal Court Judge Ronald Adrine ruled there was probable cause to bring charges of murder, involuntary manslaughter, reckless homicide, negligent homicide and dereliction of duty against Loehmann.

But the judge cautioned that his opinion was "advisory in nature" and it was ultimately up to the "discretion of the city's prosecuting authority" to file charges, the local ABC News station reported.

The ruling comes after community leaders took advantage of an unusual law which allows residents to bypass prosecutors and ask judges to issue arrest warrants.