Dallas police say multiple pieces of evidence were seized from the Mesquite home of shooting suspect Micah Xavier Johnson, an Army veteran they say shot 14 people, killing five police officers, Thursday night.

The police chief defended the decision to kill Johnson with a bomb delivered by remote-controlled robot, saying negotiations went nowhere and that officers could not approach him without putting themselves in danger.

Investigators believe that Johnson "had been practicing explosive detonations", and that he had enough explosives "to have devastating effects throughout our city and our North Texas area", Brown told CNN's "State of the Union".

The rally in Dallas followed the fatal police shootings of Philando Castile, 32, near St. Paul, Minnesota, on Wednesday, and Alton Sterling, 37, in Baton Rouge, Louisiana, on Tuesday.

When Dallas police used a bomb-carrying robot to kill the suspected sniper that had allegedly killed five police officers and wounded seven others, it opened up an ethical debate about the technology being used for crime fighting, The Associated Press reported. But until that serious discussion happens, he said he fears "we're going to continue to see this kind of tragic incident" like the Dallas attack.

Brown said that they have to do that for the sake of their families.

"Out of an abundance of caution, officers searched the garage to ensure reports of a suspicious person was thoroughly investigated", Dallas police said.

Obama, the first black US president whose term in office ends next January, said he hopes he has been able to get all Americans to understand the nation's hard legacy of race.

Police are trying to decipher what the initials mean.

Micah Johnson, 25, was identified as the suspected gunman. An armed black policeman at the crossroads between Belleview Street and Akard Street said he has received orders to block the street, but declined to disclose more information. "We just need to hear from the protestors back to us, we appreciate the work you do for us, in our right to protest". He received instruction at the Academy of Combative Warrior Arts in the Dallas suburb of Richardson about two years ago, said the school's founder and chief instructor, Justin J. Everman.

Police have said they believe Johnson had plans for a larger attack. The deaths have fomented unrest from Ferguson, Missouri, to Baltimore and heightened calls for greater accountability of police, particularly in the urban, majority-black neighborhoods they patrol. Brown also said that Johnson tried to write a message with his blood on the walls, but only managed to write "RB" before he was taken out.