DALLAS - The gunman in downtown Dallas, wearing light tan pants and a dark tan jacket, crouches behind a trash can next to the pillar of an office building.He stalks his target, who stands behind a pillar just a few feet away, then charges with a rifle. The assailant bobs and weaves - while firing - then circles the pillar and squeezes off rounds into the back of a collapsing body in a dark, short-sleeved uniform.The scene unfolds in a widely circulated video that appears to have captured one killing in Thursday's ambush of Dallas police that left five officers dead, and seven other officers and two civilians wounded. The shooter's maneuvers make clear that the gunman had extensive tactical training, military experts who reviewed the video told Reuters. The authenticity of the video could not be independently confirmed.Taken from a nearby balcony or rooftop, looking down on the bloodshed, the video captures what may be a telling moment in the deadliest attack on U.S. law enforcement since the September 11, 2001 attacks on New York and Washington.The shooting erupted near the end of a peaceful march of hundreds of people to protest the killings of two black men by police in other cities this week. Dallas police were there to ensure their safety.The ambush ended hours later when Dallas police used a robot carrying a bomb to kill a suspect that had screamed that he wanted to kill "white people, especially white officers" said Dallas Police Chief David Brown, who is black."The end is coming," Brown said the suspect warned police during the standoff, while threatening that bombs had been planted nearby.Dallas police confirmed Friday afternoon that the dead suspect is Micah X. Johnson, 25, a former member of the US Army Reserve.While authorities, including the mayor of Dallas, said they believe Johnson was the lone gunman, they are still looking into whether there were co-conspirators. They have not commented on the video and Reuters could not independently confirm that Johnson is the shooter in the video.'CERTAINLY' HAD UNDERGONE TRAININGJohnson was a private first class in the US Army Reserve from March 2009 to April 2015. He was deployed to Afghanistan from November 2013 to July 2014 and earned a number of service medals, according to US Army records.The shooter captured in the video displayed a high level of tactical sophistication, military experts told Reuters. At one point, the apparently right-handed gunman weaves to the left - firing to the left of the pillar - before circling it to the right and closing on the victim with a barrage of close-range shots.Michael Waltz, a former US Army Special Forces officer and White House aide who served in Afghanistan, said the shooter depicted had "certainly" undergone training."He is using his rifle in the way that we are trained," Waltz said. "He runs directly into fire with the police officer and then flanks him."Other indications of training, Waltz said, are the way the shooter raises and lowers the rifle to his shoulder, and appears to squeeze off two rounds at a time.Any such attack would have required extensive plotting, said Terry Turchie, former deputy assistant director of the FBI counter-terrorism division."He was obviously on a mission and had all the training he needed," Turchie said after watching the video of the shooter. "You can't carry out something like that without planning ahead of time."Dallas Mayor Mike Rawlings described Johnson this way late Friday: "This was a mobile shooter that had written manifestoes on how to shoot and move, shoot and move.""And he did that. He did his damage, but we did damage to him as well."HEARING GUNFIRE, SMELLING GUNPOWDERThe barrage of gunfire scattered large crowds of people who had come together in what promised to be a peaceful protest. At one point, two smiling officers posed with a protestor holding a sign reading, "No Justice, No peace," an image tweeted from the Dallas police Twitter account.One of the organizers of the march, Rev. Jeff Hood, was walking with a police officer when he heard what he recalled as six to eight shots, he told reporters."I heard ba-ba-ba-ba-ba," Hood said. "I saw people scramble. The officer ran towards the shots. I ran away from the shots, trying to keep people off the streets, and I was grabbing myself to see if I was shot."The march started at about 7 p.m., according to eyewitnesses at Belo Park Garden and proceeded to the nearby John F. Kennedy memorial, commemorating the assassination, also in Dallas, of the US president in 1963.A crowd of about 1,000 was disbanding, after a few speeches, when gunfire set off mayhem, said Ivan Garcia of Garland, Texas, a legal assistant for a nonprofit organization who attended the march."I could hear the gunfire and smell the gunpowder," he said.He sensed the shots came from behind and to the left of him, but couldn't be sure amid the panic."No one could tell where it was coming from a building or from the ground," he told Reuters.Garcia said most of the marchers took cover near Dallas' Old Red Courthouse and across the street in Dealey Plaza and behind the Sixth Floor Museum, the building which was the former Texas Book Depository where Lee Harvey Oswald is believed to have shot Kennedy.It initially appeared to eyewitnesses and police that multiple shooters were involved, but Johnson remains the only named suspect."It seems like there were too many shots for it just to have been one person," Garcia said.Garcia said the shooting ended about 10:45 p.m.Police killed Johnson in the parking garage of El Centro College sometime after midnight and after protracted negotiations. At around 2:00 a.m., in the middle of the night, they reported that a fifth office had died.