Dallas Police Chief David Brown told CNN on Sunday Micah X. Johnson had improvised as he used "shoot-and-move" tactics to gun down officers during a demonstration on Thursday, the deadliest day for USA law enforcement since September 11, 2001.
Dallas police headquarters and surrounding blocks were cordoned off and SWAT teams were deployed on Saturday after authorities received an anonymous threat against officers across the city, but a search for a "suspicious person" turned up no one, officials said.
Undeterred by heightened concerns about safety at protests after a lone gunman killed five police officers in Dallas on Thursday night, organisers went ahead with marches in New York, Washington DC, and other cities.
Johnson, an Army Reserve veteran, had been excited to join the military and was passionate about protecting his country, Johnson's parents said in an interview with The Blaze set to air this week.
"The military was not what Micah thought it would be", said his mother, Delphine Johnson.
Clearly exhausted, Brown talked about the number of challenges his department is facing such as violence in schools, unsafe dogs, gun control, a high rate of officer resignations and now protests.
A picture has emerged of Johnson as an angry young man with a warped interpretation of the tactics of empowerment for the black community.
Officers have also discovered a journal found in Johnson's house that is proving "hard to decipher", police said.
In Louisiana, another police officer shot and killed Alton Sterling while pinning him to the ground in a parking garage.
A protest by the Black Lives Matter group in Dallas on Thursday night provided a "target of opportunity" and he made a decision to "kill as many as he could", police said.
The fact that Johnson had material for explosives and talked of using homemade bombs during a stand-off with police before he was killed indicated he could have inflicted more damage with more time, Dallas County Judge Clay Jenkins said.
Police do not know what the letters mean but believe Johnson wrote them in his own blood.
"It was very important that we realized he may not be bluffing", Rawlings said. "To decide for themselves whether this wants to be senseless and useless, absurd moment in life or if it has a objective and a grander plan to make us a stronger country, a stronger city and a stronger world".
Some legal and civil liberties analysts said the unprecedented action threatened to disrupt the established legal order on homicides by law enforcement.
"My son is a hero", father Rick Zamarripa said.
The incident happened while hundreds of people were gathered in Dallas to protest against recent fatal police shootings.