This article is more than 4 years old

This article is more than 4 years old

Authorities in Dallas are questioning family members about how Micah Johnson was able to stockpile a vast amount of bomb-making equipment at his home, as it emerged that two more officers were wounded in last week’s shooting than was previously known.

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Five officers died and nine were wounded in the shooting last Thursday, during a protest against the deaths of African American men in confrontations with police officers. Authorities had previously stated that seven officers were injured. Two members of the public were also hurt.

Police chief David Brown said on Monday that bomb technicians had been alarmed by the scale of Johnson’s collection of explosives at the house in the suburb of Mesquite that he shared with his mother, Delphene.

Delphene Johnson has been questioned, according to Brown, but not detained. Asked by a reporter how it was possible that his family could “not know about him stockpiling weapons”, Brown said: “That’s my question.”

New details about the Dallas gunman emerged after a weekend in which hundreds of protesters in several cities were arrested during demonstrations demanding an end to what activists say is an excessive use of force by officers against African Americans.

Brown said investigators were trying to establish whether Johnson, 25, had developed expertise in bomb-making online or elsewhere.

“He knew what he was doing – this wasn’t some novice,” said Brown, adding: “We don’t think he learned it in the military.”

Police have said Johnson declared during negotiations that he planned to perpetrate many more killings, before police killed him with a remote-controlled, bomb-carrying robot. Johnson was an army veteran and had explosives that police said could have been “devastating” to the region.

Speaking at a news conference on Monday, Brown said investigators had not ruled out the possibility that Johnson was connected to a wider threat against the city.

“The concern is that we haven’t found something that’s out there,” he said.

Speaking publicly for the first time since the shootings, Johnson’s family told news outlet the Blaze that his demeanor had changed in 2015, after his discharge from the US army amid allegations of sexual assault. His mother said he had transformed from a carefree extrovert to a “hermit”. His father said he then became increasingly focused on his black heritage.

Investigators are reviewing more than 170 hours of body camera video footage and taking statements from more than 300 witnesses, Brown said. He disclosed that Johnson had left two separate inscriptions of “RB” on the walls of the El Centro community college from where he launched his attack. The significance of the letters remains unknown.

Brown also clarified that 11 officers had returned fire on Johnson with guns and two others delivered the explosives that killed him by robot.

He also voiced frustration at what he saw as the overburdening of police departments with too many problems outside of their remit when “policing was never meant to solve all of those problems”.

“We’re asking cops to do too much in this country,” he said. “Every societal failure, we put it off on the cops to solve. Not enough mental health funding, let the cops handle it. Not enough drug funding, give it to the cops … 70% of the African American community is raised by single women. Let’s give it to the cops.”

He knew what he was doing – this wasn’t some novice Police chief David Brown

The linking of a high rate of single-parent families to “societal failure” by Brown, who is himself African American, is likely to cause controversy among some black Americans.

Brown’s call for the public to adjust their expectations of officers came as protesters in Dallas, Atlanta, Baton Rouge and elsewhere continued to demand police reform.

In Baton Rouge, formerly peaceful protests over the death of Alton Sterling gave way to clashes between protesters and police wearing gas masks. Two white policemen killed Sterling, a black man, during an arrest last Tuesday that bystanders documented with cellphone videos.

Protests immediately after Sterling’s death featured gospel music and calls from his family for peace and respect for police. Over the weekend, some such protests turned violent and more than 100 people were arrested.

Facebook Twitter Pinterest Demonstrator Iesha Evans protesting the shooting death of Alton Sterling in Baton Rouge, Louisiana. Photograph: Jonathan Bachman/Reuters

The evolving protests produced a cascade of video and photography that showed arrests of journalists, the well-known activist DeRay Mckesson, and a stoic young woman in a flowing dress whose photograph has come to represent the spirit of peaceful resistance.

Baton Rouge investigators said the shift in the city’s mood was due, in part, to the arrival of protesters from other areas. They said that about half the people arrested during the weekend had come from outside the city.

In Dallas, Brown said Johnson’s aim was to punish police officers for their treatment of black people.

At the start of the new week, national political figures began to weigh in on the subject of police and race, offering a range of sentiments from criticism to consolation.

Former New York City mayor Rudy Giuliani blamed some of the strife on the Black Lives Matter movement. In an appearance Sunday on CBS’s Face the Nation he said the movement was “anti-American, and it’s racist”.

Barack Obama returned early from a trip to Spain and on Tuesday will travel to Dallas to deliver remarks at an interfaith memorial service for the officers killed. During his visit to Europe he did not speak at length on the subject of the shooting, but seemed more willing than Giuliani to distance the Dallas tragedy from the Black Lives Matter movement.

People should not, Obama said, assume the act of one possibly demented killer “speaks to some larger political statement across the country”.

Former president George W Bush and first lady Laura Bush will attend the Tuesday service, as will Vice-President Joe Biden.

Tuesday will also see the start of several days of visitations and funerals for the officers killed last week. The officers were members of the Dallas police department and the city’s transit police.