Cleveland police say two officers told a 12-year-old boy with a pellet gun three times to show his hands before he was shot to death.
Authorities on Wednesday also released video and audio evidence from the shooting.
Police identified the officers involved in the shooting of Tamir Rice as 26-year-old Timothy Loehmann, who was appointed to the force this year, and 46-year-old Frank Garmback, who's been with the department since 2008.
Police say Tamir was told to raise his hands three times, then reached into his waistband for what appeared to be a firearm. Police later determined it was an airsoft gun, which shoots small plastic pellets.
Police emphasized the video is being released as the boy's family wished, and it was not an effort to "exonerate" or condemn anyone.
The video was made available to media later.
On Tuesday evening, several hundred demonstrators marched down an exit ramp and temporarily blocked rush-hour traffic on a busy Cleveland freeway. Police diverted traffic but didn't take action against the protesters, who chanted phrases such as "Hands up, don't shoot" and "No justice, no peace."
The demonstration came as protesters across the country decried a grand jury's decision not to indict a white police officer who killed an unarmed black teenager in Ferguson, Missouri.
In a video message posted on YouTube, Anonymous charges that an "untrained rookie officer" shot Tamir "in cold blood," and asks why the officer did not Taser the child.
"Police of the United States you will learn in due time once anonymous has shut down your sites that we will not stand for your ignorant untrained rookie cops," the message said.
State Rep. Alice Reece, a Democrat from Cincinnati and the president of the Ohio Legislative Black Caucus, announced Sunday that she will introduce legislation requiring all BB guns, air rifles and airsoft guns sold in Ohio to be brightly colored or have prominent fluorescent strips.