Bomb-making material, rifles, ammunition and a combat journal have been found at the home of the suspect in the Dallas shooting, officials say.
Five police officers were killed and seven wounded in a hail of gunfire during a protest on Thursday against the shooting of black men by police.
The suspect, 25-year-old Micah Johnson, died after a long stand-off with police in central Dallas.
Mayor Mike Rawlings said officials believed he was “the lone shooter”.
“We believe now the city is safe,” the city mayor said at a news conference on Friday.
The protest in Dallas took place after this week’s deaths of Philando Castile in Minnesota and Alton Sterling in Louisiana.
Officials said on Friday that gunmen targeted police officers and civilians in separate attacks in Georgia, Missouri and Tennessee in the aftermath of the Minnesota and Louisiana deaths.
In Tennessee, a black army veteran killed a woman and also injured three other people, including a police officer, as he opened fire on a motorway on Thursday morning.
After his arrest, he told investigators he was troubled by police violence against African-Americans.
In the Georgian state capital Atlanta on Friday evening, thousands marched in protest at the recent police shootings.
The protesters are currently in a stand-off with police as they block a major road.
Police in Dallas said the bomb-making material was found when they searched Micah Johnson’s home in the suburb of Mesquite.
They also confirmed media reports that the suspect, an army veteran, was the person killed when they remotely detonated explosives they sent into the car park where he was holed up.
Mr Rawlings said the suspect was given a choice of “surrendering without harm or remaining in place”, adding that he chose “the latter”.
The authorities said that Johnson had no criminal history.
Police Chief David Brown said the suspect had told a negotiator that he had wanted to kill white people, especially white police officers, because he was angry about the recent shootings of black men by police.
Mr Brown added that the man had said he was not affiliated with any groups and had acted alone.
Police arrested three people and initially said they believed at least two snipers had caught police in crossfire, shooting some officers in the back.
‘Despicable attack’
Micah Johnson lived with his mother in Mesquite, an eastern suburb of Dallas, and had been a member of the US Army Reserve, US media reported.
Investigators later sealed off and searched his home, carrying away several bags of material.
Gunfire broke out at around 20:45 local time on Thursday (01:45 GMT Friday) as demonstrators marched through the city. Police described the ambush as carefully planned and executed.
Officers later surrounded a car park where the stand-off with the suspect took place.
