DALLAS, July 11 (UPI) -- Investigators are examining 170 hours of body cam video and interviewing more than 300 witnesses after the shootings that killed five police officers, the city's police chief said in a news conference Monday.
Brown said, "We're going to turn over every rock ... until I'm satisfied that this was the lone person."
Brown also revised the number of officers injured from seven to nine: two with the Dallas Police Department, two Dallas Area Rapid Transit officers and two with the El Centro police department. Two civilians were also hurt.
In a separate news conference at Parkland Hospital, doctors spoke about treating the officers, some of whom died.
The police chief said 11 officers shot back at suspect Micah Johnson, 25, and two used the explosive robot device that killed him while he was inside the second floor of a parking garage in downtown Dallas.
During negotiations, Brown said Johnson asked how many officers were killed.
"The person obviously had some delusion," he said. "The person was also very committed to killing officers."
He defended the use of the robot, saying Johnson "already killed us in a grave way, and officers were in surgery that didn't make it." And "this wasn't an ethical dilemma for me."
The robot was purchased in 2008 for $151,000, Brown said.
Brown said investigators were trying to determine Johnson's plans.
"We don't know the scope of his plans yet," chief says. "There was a large stockpile" of bomb-making materials in his home.
The police department is helping prepare for Tuesday's memorial service that will include President Barack Obama.
Dallas police will not be the lead agency on security plans. He said Arlington police will work with the Secret Service on security during Obama's visit.
At Parkland, doctors recounted the night.
Emergency-room caregivers are used to dealing with trauma, but the situation rocked some "to their core that I thought were unshakable," said Dr. Alex Eastman.
Dr. Brian Williams, one of the trauma surgeons treating officers, became emotional during the news conference.
"I think about it every day that I was unable to save those cops when they came here that night," he said.
William, who is black, urged an end to the violence.
"I stand with the Dallas Police Department," Williams said. "I stand with law enforcement all over this country. I abhor what has been done to these officers and I grieve with their families."
He noted, "The problem is the lack of open discussion about the impact of race relations in this country. This killing has to stop."
William said he is trying to teach his young daughter how to interact with police officers. When he sees officers in restaurants, he says he'll pick up their tabs.