This file photo obtained July 8, 2016 shows shows Micah Xavier Johnson.
This file photo obtained July 8, 2016 shows shows Micah Xavier Johnson.

Investigators in Dallas, Texas, are continuing to probe the background of the Dallas gunman who killed five policemen last week.

"Until we search every lead," Chief David Brown said Monday, "I'm not going to be satisfied" in ruling out that others may have been involved, to "make sure we don't leave anything uninvestigated."

He said police are examining 170 hours of video from cameras worn by officers, as well as footage from dashboard cameras inside police cars and video from retail store security cameras along the streets where the attacks on police occurred. In addition to the five officers killed in the incident, nine other police officers and two civilians were wounded.

Brown said 11 police officers fired weapons at Micah Johnson, and two others launched the $151,000 robot used to kill the suspect.

The police chief said it was "not an ethical dilemma for me" to use the robot to kill Johnson, apparently the first time a U.S. police force has used such a tactic to end a criminal rampage.

"I'd do it again to save officers' lives," Brown said.

"We're convinced that this suspect had other plans, and thought that what he was doing was righteous and believed that he was going to target law enforcement, make us pay for what he saw as law enforcement's efforts to punish people of color," Brown said.

Dallas police chief David Brown, front, and Dallas
FILE - Dallas police chief David Brown, front, and Dallas mayor Mike Rawlings, rear, talk with the media during a news conference, in Dallas.

Bomb-making materials and a rambling journal were found at Johnson's home during a search.

"The materials were such that it was large enough to have devastating effects throughout our city and our North Texas area," Brown said.

U.S. law enforcement officials said Johnson did not have a criminal record.

In an emailed statement, the U.S. Army said Johnson was a carpentry and masonry specialist during his time in the service from March 2009 to April 2015. He was deployed to Afghanistan from November 2013 to July 2014.

Johnson left Afghanistan when a fellow soldier brought sexual harassment charges against him, saying he needed mental health counseling.

A surgeon's plea

During a news conference Monday, the black doctor who treated dying Dallas police officers nearly broke down in tears.

"This killing, it has to stop," Brian Williams, a trauma surgeon at Parkland Memorial Hospital, pleaded. "Black men dying and being forgotten. People retaliating against the people that are sworn to defend us. We have to come together and end all of this."

Williams said as a black man in America he faces a dichotomy of standing with law enforcement — one of his colleagues is a doctor who is also a police officer — but also feeling angst and fear any time he passes a police officer.

"I understand the anger and the frustration and distrust of law enforcement, but they are not the problem," he said.

"It's not just about that one night. It's about the racial undertones that affect all of this,'' Williams said.

U.S. President Barack Obama delivers remarks on th
FILE - U.S. President Barack Obama delivers remarks on the police shootings in Dallas after meeting with EU leaders at the NATO Summit in Warsaw, Poland, July 8, 2016.

Obama visit

U.S. President Barack Obama, who is visiting Dallas on Tuesday for a memorial service for the five policemen who were killed, has called on Americans to mutually respect fellow citizens, after the recent days of violence.

"Maintaining a truthful and serious and respectful tone is going to help mobilize American society to bring about real change and that is our ultimate objective," Obama said Sunday. 

"I would like all sides to listen to each other," Obama said in reference to protesters, such as those involved with the Black Lives Matter movement and police organizations throughout the United States.

Obama said violence against police by anyone concerned about fairness in the criminal justice system does "a disservice to the cause."

A policeman visits a makeshift memorial at the Dal
A policeman visits a makeshift memorial at the Dallas police headquarters, July 11, 2016, in Dallas.

Demonstrations have been held in many cities across the country in recent days, with some confrontations between protesters and police, who have made scores of arrests.

Johnson changed after military

The parents of the 25-year-old black Army veteran say he was a changed man after being discharged from the military last year.

Delphine Johnson, the mother of the shooter, Micah Johnson, told TheBlaze media site that her son changed from a fun-loving extrovert into a "hermit" after serving six years as a U.S. Army reservist, including a seven-month deployment to Afghanistan, where the United States has been engaged militarily for 15 years.

She and Johnson's father, James Johnson, said they could not pinpoint any particular incident during his time in the Army as transforming their son. Delphine Johnson said her son "loved his country" and "wanted to protect his country."

Micah Johnson, the suspect in the Dallas shooting, is seen in this undated Facebook post.

Yet she said, "The military was not what Micah thought it would be. He was very disappointed, very disappointed. But it may be that the ideal that he thought of our government, what he thought the military represented, it just didn't live up to his expectations."

Johnson's father said that his son, upon leaving the military, immersed himself in studying black history and learning about his heritage. The younger Johnson’s Facebook page, since deleted, featured pictures of a "black power" fist and a red, black and green Pan-African flag, both black empowerment symbols. The Facebook page also showed he supported the New Black Panther Party, a group that has called for violence against whites.

The family said he never exhibited any hatred of whites, but he told police negotiators during last Thursday's rampage that he wanted to kill as many white policemen as he could in revenge for the police shooting of two African-American men last week, one in Louisiana and one in Minnesota. The Dallas standoff ended when police detonated a bomb attached to a robot they sent near where he was carrying out his assault.

Johnson's father said he was at a loss to explain his son's actions.

"I don't know what to say to anybody to make anything better. I didn't see it coming," Johnson said. "I love my son with all my heart. I hate what he did."