The suspect, named by officials as 25-year-old Micah Johnson, died after a stand-off with police following the attacks which left five officers dead and seven injured during protests over two recent fatal police shootings of black men.
Police chief David Brown said at a news conference that Johnson told negotiators he was acting alone and was unaffiliated with any group.
Mr Brown said officers killed him with a robot-delivered bomb after the failure of hours of negotiations in a city-centre parking garage during which he made threats about bombs.
Police sealed off the city centre but later tweeted that primary and secondary sweeps for explosives were complete and no explosives had been found.
Although Johnson apparently said he acted alone, it remains unclear if that was the case. Mr Brown said earlier that three other suspects were in custody, but he declined to discuss those detentions and said police do not know if investigators have accounted for all participants in the attack.
The gunfire broke out as hundreds of people gathered to protest over incidents in Baton Rouge, Louisiana, and St Paul, Minnesota.
The Dallas shootings killed four city police officers and one officer with Dallas Area Rapid Transit, which operates buses and the state of Texas’s largest municipal rail system.
Mr Brown told reporters snipers fired “ambush style” on the officers, and Dallas Mayor Mike Rawlings said two members of the public were wounded in the gunfire.
Video footage from the scene showed protesters marching about half a mile from City Hall, when the shots erupted and the crowd scattered, seeking cover.
Mr Brown said that it appeared the shooters “planned to injure and kill as many officers as they could”.
The attacks made Thursday the deadliest day in US law enforcement since the September 11 terrorist attacks in 2001, according to the National Law Enforcement Officers Memorial Fund.
The search for the shooters stretched throughout the city centre, an area of hotels, restaurants, businesses, and some residential apartments. The area is a few blocks away from Dealey Plaza, where President John F Kennedy was assassinated in 1963.
The scene was chaotic, with helicopters hovering overhead and officers with automatic rifles on street corners.
“Everyone just started running,” Devante Odom, 21, told The Dallas Morning News. “We lost touch with two of our friends just trying to get out of there.”
Carlos Harris, who lives nearby, told the newspaper that the shooters “were strategic. It was tap, tap, pause. Tap, tap, pause”.
Video posted on social media appeared to show a gunman at ground level exchanging fire with a police officer who was then felled.
Authorities have not determined whether any protesters were involved with or were complicit in the attack.
One woman was taken into custody in the same parking garage as the stand-off, Mr Brown said. Two others were taken into custody during a traffic stop.
Mr Rawlings said at a news conference that authorities were asking people to stay away from the city centre: “This is still an active crime scene. We’re determining how big that crime scene is.”
A map was being posted online showing an area where people should avoid, he said.
Theresa Williams said one of the injured civilians was her sister, 37-year-old Shetamia Taylor.
Ms Williams said her sister was at the protests with her four sons, aged 12 to 17. When the shooting began, Ms Taylor threw herself over her sons, Ms Williams said. She was undergoing surgery after being shot in the right calf.
Texas governor Greg Abbott released a statement saying he had directed the Texas Department of Public Safety director to offer “whatever assistance the City of Dallas needs at this time”. “In times like this we must remember — and emphasise — the importance of uniting as Americans,” Mr Abbott said.
A man identified by police on Twitter as a suspect in the attack has said he turned himself in and was released.
Dallas Police put out a photo on its Twitter account of a man wearing a camouflage shirt and holding a rifle with the message: “This is one of our suspects. Please help us find him!”
The man, Mark Hughes, told Dallas TV station KTVT he “flagged down a police officer” immediately after finding out he was a suspect.
He said police lied during a 30-minute interrogation, saying they had video of him shooting.
Videos online show Mr Hughes walking peacefully in the shooting and later turning over his gun to a police officer.