Homeland Security Secretary Jeh Johnson said at a news conference Friday in New York City that the lone suspect, Micah Xavier Johnson, doesn't appear to have had any known "links to or inspiration from any international terrorist organization."
Gunmen shot and killed five police officers and wounded seven others during a protest over fatal police shootings of black men in other states, authorities said. It appeared to be the deadliest day for U.S. law enforcement since the 2001 terrorist attacks.
Thursday's bloodshed, which unfolded just a few blocks from where President John F. Kennedy was slain in 1963, also evoked the trauma of the nation's tumultuous civil rights era.
Police Chief David Brown blamed "snipers" and said three suspects were in custody. Mayor Mike Rawlings said a fourth was slain by police in a downtown parking garage where he had exchanged gunfire with authorities.
"We don't exactly know the last moments of his death, but explosives did blast him out," Rawlings told The Associated Press. Police said the man had told negotiators he intended to hurt more law enforcement officials.
Police did not identify any of the suspects. The police chief said the dead suspect had declared before his death that he was upset about recent shootings and wanted to kill whites.
The shooting began about 8:45 p.m. Thursday while hundreds of people were gathered to protest the week's fatal police shootings in Baton Rouge, Louisiana, and suburban St. Paul, Minnesota. Brown told reporters the snipers fired "ambush style" on the officers. Two civilians were also wounded, Rawlings said.
Brown said it appeared the shooters "planned to injure and kill as many officers as they could." Video from the scene showed protesters marching along a downtown street about half a mile from City Hall when shots erupted and the crowd scattered, seeking cover.
"I think the biggest thing that we've had something like this is when JFK died," resident Jalisa Jackson downtown said early Friday as she struggled to fathom the still-unsettled situation. Officers crouched beside vehicles, SWAT team armored vehicles arrived and a helicopter hovered overhead.
Demonstrations were held in several other U.S. cities Thursday night to protest the police killings of two more black men: A Minnesota officer on Wednesday fatally shot Philando Castile while he was in a car with a woman and a child, the shooting's aftermath livestreamed in a widely shared Facebook video. A day earlier, Alton Sterling was shot in Louisiana after being pinned to the pavement by two white officers. That, too, was captured on a cellphone video.
Thursday's shootings occurred in area of hotels, restaurants, businesses and some residential apartments only a few blocks from Dealey Plaza, the landmark made famous by the Kennedy assassination.
The scene was chaotic, with officers with automatic rifles on the 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 downtown, told the newspaper that the shooters "were strategic. It was tap, tap, pause. Tap, tap pause," he said.
Brown said police don't have a motivation for the attacks or any information on the suspects. He said they "triangulated" in the downtown area where the protesters were marching and had "some knowledge of the route" they would take.
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 the attack and were not certain early Friday that all suspects had been located, Brown said.
Rawlings said authorities would likely ask some people to stay away from downtown Dallas on Friday. Rawlings said a map would be posted online showing an area where people should avoid on Friday.
Early Friday morning, dozens of officers filled the corridor of the emergency room at Baylor Medical Center, where other wounded officers were taken. The mayor and police chief were seen arriving there.
Four of the officers who were killed were with the Dallas Police Department, a spokesman said. One was a Dallas Area Rapid Transit officer. The agency said in a statement that 43-year-old officer Brent Thompson was the first officer killed in the line of duty since the agency formed a police department in 1989.
"Our hearts are broken," the statement said.
Theresa Williams told The Associated Press that one of the wounded civilians was her sister, 37-year-old Shetamia Taylor.
The identity of the other civilian casualty was not immediately known. Mayor Rawlings said he did not believe either had life-threatening injuries.
Williams said her sister was shot in the right calf and had thrown herself over her four sons, ages 12 to 17, when the shooting began.
Texas Gov. 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 emphasize -- the importance of uniting as Americans," Abbott said.
Other protests across the U.S. on Thursday were peaceful. In midtown Manhattan, protesters first gathered in Union Square Park where they chanted "The people united, never be divided!" and "What do we want? Justice. When do we want it? Now!" In Minnesota, where Castile was shot, hundreds of protesters marched in the rain from a vigil to the governor's official residence. Protesters also marched in Atlanta, Chicago and Philadelphia.
President Barack Obama said America is "horrified" over the shootings and there's no possible justification for the attacks.
He called them "vicious, calculated and despicable."
Speaking from Warsaw, Poland, where he was meeting with leaders of the European Union and attending a NATO summit, the president said justice will be done and he's asking all Americans to pray for the fallen officers and their families.
He also said the nation should express its gratitude to those serving in law enforcement.
The National Law Enforcement Officers Memorial Fund, which tracks on-duty deaths, said it was the deadliest day for U.S. police since Sept. 11.