DALLAS — A military veteran who said his goal was to kill white police officers is believed to have acted alone when he opened fire with an AR-15 rifle in Dallas, leaving five officers dead and seven wounded in the most deadly assault on American police since the terrorist attacks of Sept. 11, 2001.

Dallas Police Chief David O. Brown said Friday that officers killed the suspect, Micah X. Johnson, 25, with a remote-controlled explosive delivered by a robot. It was apparently the first use of a robot by US officials to kill a suspect during a siege outside a war zone.

Searching Johnson’s home in the Dallas suburb of Mesquite on Friday, “detectives found bomb-making materials, ballistic vests, rifles, ammunition, and a personal journal of combat tactics,’’ police said in a statement.

Brown declined to identify three people who were arrested after the Thursday attack, or to say what role they might have played in the plot. During unsuccessful talks with police negotiators, Johnson claimed he acted alone, but “we’re not satisfied that we’ve exhausted every lead,’’ Brown said.

More shooting attacks on police were reported Thursday and Friday, including shootings in Georgia, Missouri, and Tennessee. In Tennessee, a man accused of shooting indiscriminately at passing cars and police on a highway told investigators he was angry about police violence against African-Americans. One woman died in the Tennessee attack and three others, including one police officer, were injured in the rampage.

In Dallas, the gunfire erupted just before 9 p.m. Thursday, aimed at officers who were monitoring a demonstration by thousands of people protesting the fatal shootings earlier in the week of black men by police officers in Minnesota and Louisiana.

The peaceful march quickly gave way to chaos and screams as terrified people, including families with children, ran for cover, while police officers with guns drawn ran the other way, toward the shooting, and returned fire.

Two civilians were wounded by gunfire.

During a standoff that lasted for hours after the attack, the sniper said he wanted to kill more officers, claimed — apparently falsely — to have planted explosives in the area, and told police negotiators that “he was upset about Black Lives Matter,’’ Brown said.

“He said he was upset about the recent police shootings,’’ Brown said. “The suspect said he was upset at white people. The suspect stated he wanted to kill white people, especially white officers.’’

Mayor Mike Rawlings said the gunman wore a protective vest and used an AR-15 semiautomatic rifle, a weapon similar to the one fired in June in the attack on an Orlando nightclub that killed 49 people, the Associated Press reported.

With the shooter dead, Rawlings declared Friday that the city was safe and ‘‘we can move on to healing.’’

At first, Dallas officials said that multiple snipers had carried out a coordinated ambush of the officers — some of whom were shot in the back, the chief said — but later, Homeland Security Secretary Jeh Johnson said it appeared that the suspect killed by police was the sole gunman.

Micah Johnson, an Army Reserve veteran who served in Afghanistan from November 2013 to July 2014, had no criminal record, the police said.

Investigators have not turned up any evidence that Johnson, who was black, had direct ties to the Black Lives Matter movement or to other political groups. But his Facebook page indicated support for the New Black Panther Party, a militant group that has advocated violence against whites.

The sequence of events this week tore at a nation already deeply divided over questions of policing and race, moving from anger and despair over shootings of black men by police to officers being targeted in apparent retaliation.

“All I know is that this must stop, this divisiveness between our police and our citizens,’’ Brown said.

Just hours after President Obama, reacting to video recordings of the shootings by police in Baton Rouge, La., and Falcon Heights, Minn., spoke in anguished terms about the disparate treatment of the races by the criminal justice system, he felt compelled to speak again, this time about the people who attacked officers.

“We will learn more, undoubtedly, about their twisted motivations, but let’s be clear: There are no possible justifications for these attacks or any violence towards law enforcement,’’ he told reporters Friday in Warsaw, where he was attending a NATO summit meeting.

He plans to cut short his European trip and visit Dallas early next week.

Attorney General Loretta E. Lynch said she is “deeply grateful for the difficult and dangerous work’’ done by police. To the protesters, she said, “Do not be discouraged by those who would use your lawful actions as a cover for their heinous violence.’’

Civilians also have been caught in the fray of the latest attacks nationwide. The woman who died in Tennessee was a newspaper carrier driving down the highway.

The Tennessee Bureau of Investigation said that the preliminary investigation revealed that the suspect, 37-year-old former soldier Lakeem Keon Scott, who is black, was troubled by the incidents in other states, hundreds of miles away. All those shot were white, police said.

Scott was struck by the officers returning fire. He remains in the hospital in serious but stable condition. He has not been charged.

His cousin Sarah Scott said she is so close to him he called her ‘‘sister.’’ She said she is shocked by the allegation he was enraged by police violence against African-Americans.

In south Georgia, police said one officer was ambushed Friday when he came to an apartment complex to investigate a report of a break-in. Another officer was fired upon by a motorist north of Atlanta.

Just outside St. Louis, police said an officer was ambushed during a traffic stop.

The Dallas shootings, only a few blocks from Dealey Plaza, where President John F. Kennedy was assassinated in 1963, hit one of the city’s busiest areas, filled with hotels, restaurants, and government offices. Hundreds of people were effectively trapped indoors for much of the night.

As shots boomed in quick succession between office towers illuminated by the flashing lights of police cars, bystanders captured extraordinary video of the shootout, with some officers taking shelter behind patrol cars and pillars, and tending to their fallen comrades.

After Johnson was cornered on the second floor of a parking garage, negotiators spent hours trying to get him to surrender, Brown said, but he “told our negotiators that the end is coming and he’s going to hurt and kill more of us, meaning law enforcement, and that there are bombs all over the place in this garage and downtown.’’

“The negotiations broke down, and we had an exchange of gunfire with the suspect,’’ the chief said. “We saw no other option but to use our bomb robot and place a device on its extension for it to detonate where the suspect was.’’

The three other suspects were a woman who was taken from the garage and two others who were taken in for questioning after a traffic stop, but they were not providing much information, the chief said.

“We just are not getting the cooperation we’d like, to know that answer of why, the motivation, who they are,’’ he said.

Police said that four of the dead were Dallas police officers and that one was from the Dallas Area Rapid Transit force.

The transit agency identified its officer as Brent Thompson, 43. He joined in 2009 and was the first DART officer to be killed in the line of duty.

Another of the officers killed was identified by his family, on social media, as Patrick Zamarripa. “Need prayers to get through this,’’ Zamarripa’s father, Rick Zamarripa, said in a Facebook post from Parkland Hospital on Friday.

The other Dallas officers were Lorne Ahrens, Michael Krol, and Michael Smith.

Krol, a native of Michigan, lived during his high school years in East Longmeadow in Western Massachusetts. Relatives said Krol was single but had a girlfriend in Dallas.

Johnson served as a private in the Army Reserve from March 2009 to April 2015, according to records released by the Pentagon.

Jane E. Bishkin, a Dallas lawyer who represents five of the wounded officers, said that they were expected to recover but that one of the five, a woman, had suffered a serious injury to her left arm and might be disabled as a result.

The area remained off-limits to civilians through the day Friday, as investigators combed through a crime scene that extended for blocks.

Brown said the gunman appeared to have some knowledge of the demonstration march route. “We have yet to determine whether or not there was some complicity with the planning of this, but we will be pursuing that,’’ he said.

Jeff Hood, a minister who took part in the march, said he had seen two officers felled by gunfire. ‘‘This is a devastating time for us as activists and organizers. We cannot bring about justice through violence,’’ he said.