Johnson, who was apparently wounded in a shootout with police, wrote the letters "RB" and other markings.
Dallas police say the gunman staged a furious ambush-style attack in Dallas at a rally held to protest this week's fatal shooting of two black men by police in other states.
Hundreds of people marched through Times Square in New York City to protest the deaths this week of Philando Castile and Alton.
'I don't think that the answer is to continue to shoot our young black men in the United States. It's just unfortunate. Also, obviously violence is not the answer of solving it. The shooting in Dallas was very sad. No one deserves to lose their life, doesn't matter what color they are, where they're from. We're all human'
The man, named as Micah Xavier Johnson, a 25-year-old Army reservist, was responsible for the deadliest attacks on law enforcers in America since 9/11 - shooting 12, killing 5.
Sleuths of security forces and secret service agents have converged on the city and huge make-shift iron walls have come up several blocks around the convention's location.
The US president paid tributes to the five police officers killed in the deadly sniper attack in Dallas.
As the gunfire stopped at a parking garage in downtown Dallas, where five police officers were killed and seven others injured in "ambush style" firing during a protest against the fatal police shootings of black men this week, a new trend in crime-fighting crawled into spotlight.