Dallas – Dallas Police Chief David Brown on Monday defended the decision to use a remote-controlled robot to deliver the bomb that killed the Dallas gunman, identified as Micah Johnson, after a shoot-out.Brown clarified on Monday where Johnson was killed with a bomb delivered by a remote-controlled robot, saying that it happened on the second floor of El Centro College, not a parking garage as authorities previously described. Brown did not provide more details, including the locations of the negotiations that came before the bomb.Two El Centro students hid in the building overnight, Brown said, because they were afraid to come out until the shooting stopped. Police got them out of the building on Friday morning. Two officers from El Centro were injured.The police chief again defended the decision to use the robot, saying he had "already killed us in a grave way, and officers were in surgery that didn't make it"."This wasn't an ethical dilemma for me," Brown said. "I'd do it again ... to save our officers' lives."Johnson, 25, fatally shot five officers in Thursday's attack while hundreds of people were gathered in downtown Dallas to protest recent fatal police shootings, and wounded at least nine officers and two civilians.His mother, Delphine Johnson, told The Blaze website in an interview that her son wanted to be a police officer as a child. His six years in the Army Reserve, including a tour in Afghanistan, were "not what Micah thought it would be ... what he thought the military represented, it just didn't live up to his expectations".According to the military lawyer who represented him, Johnson was accused of sexually harassing a female soldier while deployed.'Mental help'Johnson's time in the Army was marked by a sexual harassment accusation in May 2014 while in Afghanistan.The Army sent him stateside, recommending an "other than honourable discharge" – which is "highly unusual" because counselling is usually ordered before more drastic steps are taken, said Bradford Glendening, the military lawyer who represented him."In his case, it was apparently so egregious, it was not just the act itself," Glendening told The Associated Press. "I'm sure that this guy was the black sheep of his unit."According to a court filing, the victim said she wanted Johnson to "receive mental help," and sought a protective order to keep him away from her and her family. Johnson was ordered to avoid all contact with her. It was unclear whether Johnson ever received counselling.Brown provided details of authorities' negotiations with Johnson on Sunday on CNN's State of the Union, saying Johnson "obviously had some delusion" as evidenced by rambling journal entries and writing "RB" and other markings in blood on walls near the shooting site – the meanings of which were unclear and being looked at by investigators.Brown also said that Johnson, who insisted on speaking with a black negotiator, laughed at authorities, sang and at one point asked how many officers he had shot.Plans for larger assaultAuthorities have said Johnson had plans for a larger assault, possessed enough explosive material to inflict far greater harm and kept a journal of combat tactics.Eleven officers fired at Johnson and two used an explosive device, Brown said, adding that the investigation will involve more than 170 hours of body camera footage and "countless hours" of dash cam video."Bravery is not a strong enough word to describe what they did that day," Brown said of officers' response to Thursday's events.Federal agents are trying to trace the origin of the weapons used by Johnson, including a semi-automatic rifle. About 30 agents are involved in identifying bullet casings, said William Temple, the Dallas agent in charge for the Bureau of Alcohol, Tobacco, Firearms and Explosives.Surgeons at Parkland Memorial Hospital spoke on Monday afternoon about treating some of the victims. Dr Brian H Williams, who is African American, said that not being able to save some of the officers weighs on his mind "constantly".He also added: "It has to stop. Black men dying and being forgotten. People retaliating against the people sworn to protect us."Dr Alex Eastman, the director of the hospital's trauma centre who is also a deputy medical director with the city's police department, said the shootings "rocked some guys to their core that I thought were unshakable".Memorial serviceThe shootings, just a few blocks from where President John F. Kennedy was slain in 1963, marked the deadliest day for US law enforcement since the September 11, 2001, terrorist attacks.This attack began during protests over the police killings of Philando Castile, who was fatally shot near St Paul, Minnesota, and Alton Sterling, who was shot in Baton Rouge, Louisiana, after being pinned to the pavement by two white officers.Video from Dallas showed protesters marching along a downtown street about 800m from City Hall when shots erupted and the crowd scattered, seeking cover.Speakers at a candlelight vigil held on Monday night in downtown Dallas for the officers who were killed included police officials and friends of the slain officers. Portraits of the officers were propped up in front of the speaker's platform.A memorial service is planned on Tuesday, with President Barack Obama, Vice President Joe Biden and former President George W Bush and his wife, Laura, scheduled to attend.Texas Governor Greg Abbott announced on Monday he will not be able to make it because he'll be undergoing skin grafts on his feet after suffering second- and third-degree burns on his feet and both legs below the knees. His wife, Cecilia Abbott, will take his place.