DALLAS — The gunman who killed five police officers here studied the “shoot-and-move” combat tactics that he apparently used, writing about it extensively in a journal that detectives are poring over, a senior local official said Saturday. He also practiced military exercises in his backyard.
Investigators found the “fairly voluminous” journal in the home of Micah X. Johnson, the sniper who shot at officers in downtown Dallas on Thursday night, said Clay Jenkins, Dallas County’s chief executive.
While the journal did not specifically lay out plans for that assault, he said, it showed how the gunman planned to adapt the combat tactic.
Johnson’s journal described “what we call ‘shoot and move’ tactics — ways to fire on a target and then move quickly and get into position at another location to inflict more damage on targets without them being able to ascertain where the shots are coming from,” Jenkins said.
The tactic described reflects the approach Johnson used in Dallas, moving from one vantage point to another, leading the police to believe at first that there were multiple gunmen.
“It’s talking not only about how to kill, but how to keep from being killed,” said Jenkins, who said he had not read the journal itself but that he had read and heard summaries.
Dallas police said Saturday that they had tightened security as a precaution after receiving an anonymous threat.
More than 20 blocks of downtown remained cordoned off as investigators worked for a second day on piecing together the details of the attack, an investigation that has included more than 200 interviews.
President Obama is cutting short a visit to Europe and will visit Dallas early next week, the White House said. Obama, speaking Friday in Warsaw, condemned what happened in Dallas as “a vicious, calculated and despicable attack on law enforcement.”
Obama expressed optimism that the strife in the last week had not left the United States as racially divided as it was during the 1960s.
“There is sorrow, there is anger, there is confusion about next steps,” Obama said. “But there is unity in recognizing that this is not how we want our communities to operate. This is not who we want to be as Americans.”
‘‘You’re not seeing riots and you’re not seeing police going after people who are protesting peacefully,’’ he said. ‘‘You've seen almost uniformly peaceful protests and you've seen, uniformly, police handling those protests with professionalism.’’
Obama said the Dallas shooter was a ‘‘demented individual’’ who does not represent black Americans any more than a white man accused of killing blacks at a church in Charleston, S.C., represents whites.
As Dallas grieved, and as the United States reeled from a week that brought two high-profile killings of black men by police and then the deadly attack on the Dallas officers, discord continued.
Demonstrations resumed Saturday in Louisiana and Minnesota, the states where Alton Sterling and Philando Castile were killed by police officers, and in other cities.
The organizers of Thursday’s protest at which the Dallas officers were killed, along with Black Lives Matter Network, have condemned the violence.
“This is a tragedy — both for those who have been impacted by yesterday’s attack and for our democracy,” the Black Lives Matter Network said. “There are some who would use these events to stifle a movement for change and quicken the demise of a vibrant discourse on the human rights of black Americans. We should reject all of this.”
The Dallas massacre was a ‘‘horrific, despicable’’ act, the Rev. Al Sharpton said Saturday, but he also argued that police officers must be prosecuted when they are involved in the deaths of innocent people.
‘‘We are not antipolice, we are not trying to kill police,’’ the civil rights activist told a crowd of over 200 during his weekly address at the National Action Network’s ‘‘House of Justice,’’ in Harlem. ‘‘We’re trying to stop the killing of us by those that are bad police,’’ he added.
Sharpton, who has protested against alleged police misconduct for decades, said the killings of black men in Louisiana and Minnesota, as well as the Dallas shootings, have put the nation at a ‘‘tipping point.’’
‘‘The question is,’’ he said, ‘‘is it a tipping point for the country to move forward or will it be a tipping point for the country to go backwards?’’
Neighbors have told investigators that Johnson, a 25-year-old Army Reserve veteran who served in Afghanistan, had an interest in weapons, and officials have said that a cache of arms, ammunition, bomb-making material, and body armor were found in his home in Mesquite, a Dallas suburb.
After the shooting, he was cornered by police, who eventually killed him early Friday.
The journal “shows that he’s well prepared,” said Jenkins, who as county judge is both the county’s top executive and its director of homeland security and emergency management.
“He had an interest, according to his neighbors, in weaponry,’’ Jenkins said. “He was doing military exercises, according to one neighbor witness, in his backyard for a couple weeks before this.”
Johnson served in an engineering brigade, but he would have had combat training. During the attack, he used a semiautomatic SKS rifle and a high-capacity handgun.
Police Chief David O. Brown has said that during the standoff with police, the suspect said he had set out to kill officers, particularly white ones.
The dead officers have been identified by law enforcement officials and family members as Lorne Ahrens, Michael Krol, Michael J. Smith, and Patrick Zamarripa, along with transit police officer Brent Thompson. Krol, a Michigan native, spent part of his youth in East Longmeadow, Mass.
Two squad cars outside police headquarters have become memorials, covered in flowers, balloons, posters, and handwritten notes.
“We’re all human here, and I think that people feel each other’s pain,” Mayor Mike Rawlings said Saturday as he visited the headquarters.