Bomb-making material, rifles, ammunition and combat journal have been found at the home of the main suspect in the Dallas shooting.
Police said the material was found when they searched the home of Micah Johnson in the Dallas suburb of Mesquite.
The gunman, killed in a stand-off with police, said he wanted to kill white people, especially officers, the city’s police chief said.
Johnson, 25, said he was upset about the recent police shootings of black people, Police Chief David Brown said.
Five police officers were killed and seven wounded during a march against the shooting of black men by police.
Three people are in custody but it is not clear if there were other gunmen.
The protest in Dallas took place after this week’s deaths of Philando Castile in Minnesota and Alton Sterling in Louisiana.
Mr Brown said the suspect had been killed when police used explosives placed by a robot to end a tense stand-off in a building where he had taken cover. Before that, he had spoken to a negotiator.
“He said he was upset about Black Lives Matter [protest movement]; he said he was upset about the recent police shootings,” Mr Brown told a news conference.
“The suspect said he was upset at white people. The suspect stated that he wanted to kill white people, especially white officers.”
Mr Brown added that the man had said he was not affiliated with any groups and he had acted alone.
However, speaking earlier on Friday, Mr Brown had said two snipers had fired from “elevated positions”, shooting some officers in the back.
The White House later said investigators had ruled out any connection to a terrorist organisation.
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. 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.
Dallas Mayor Mike Rawlings said that two civilians were also injured in the shootings. One underwent surgery for a gunshot wound in the leg.
He said the three suspects in custody so far were not co-operating and were “tight-lipped”.
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.
