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.
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Police said the bomb-making material was found when they searched Micah Johnson's home in the Dallas 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.
Image copyright FacebookMr 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 he 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.
'Unfathomable tragedy'
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.
Image copyright Getty ImagesGunfire broke out at around 20:45 local time on Thursday (01:45 GMT Friday) as demonstrators marched through the city. There was panic as people scrambled for cover. Police described the ambush as carefully planned and executed.
Officers later surrounded a car park near El Centro College where the stand-off with the suspect took place.
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Two civilians were also injured in the shootings. One underwent surgery for a gunshot wound in the leg.
President Barack Obama, who is attending a Nato summit in Poland, called it a "vicious, calculated and despicable attack on law enforcement".
He later ordered all flags on public buildings to be flown at half-mast.
Mr Obama will visit Dallas early next week, and "will continue the work to bring people together to support our police officers and communities, and find common ground by discussing policy ideas for addressing the persistent racial disparities in our criminal justice system", the White House said.
US Attorney General Loretta Lynch described events in Dallas as "an unfathomable tragedy".
"Americans across our country are feeling a sense of helplessness, of uncertainty and of fear," she said.
"These feelings are understandable and they are justified. But the answer must not be violence."
The Dallas attack marks the deadliest day for US law enforcement officers since the 9/11 attacks in 2001.
The protest march that wound its way through the city was among several held across the US over the police use of lethal force against African Americans.
Philando Castile was shot dead after being stopped in his car by police in St Paul, Minnesota , on Wednesday.
Alton Sterling was killed by police a day earlier in Baton Rouge, Louisiana.
Both incidents were captured on video, reigniting what has become a national debate.
Shock in Minnesota: Barbara Plett Usher, BBC News, St Paul
The disturbing video of Philando Castile's dying moments galvanised Pastor Thomas Mite to speak out more publicly against police killings.
But, he said, the Dallas attacks have been a setback to the activists' push for change and justice. The mood in St Paul was sober as residents absorbed this shock turn of events and what it might mean.
"Dallas is 'bigger than Philando," Mr Castile's girlfriend said to media at the protest site outside the governor's mansion, adding: "This is bigger than all of us."
"I hope there is justice for the people who did the killing in Dallas and for the person who did the killing here," said one of the protesters. Then he returned to abiding fears.
"I hope no cop gets angry here and tries to take revenge."
"The police will protect themselves,' Pastor Mite mused. "They have a right to go home safely at night, but they'll come out with riot gear and shields. This is bad."