Dallas - The black Army veteran who killed five Dallas police officers donned a protective vest and used a military-style semi-automatic rifle in the sniper slayings, officials said on Saturday. It was an attack that layered new anxiety onto a nation already divided about guns and how police treat African-Americans.Micah Johnson was killed by a robot-delivered bomb on Thursday after the shootings, which marked the deadliest day for US law enforcement since the September 11 2001 terrorist attacks.In all, 12 officers were shot just a few blocks from where President John F Kennedy was killed in 1963.Personal arsenalIn Georgia, Missouri and Tennessee, authorities said gun-wielding civilians also shot officers in individual attacks that came after two black men were killed this week in Louisiana and Minnesota. Two officers were wounded, one critically."America is weeping," said Republican GK Butterfield, the head of the Congressional Black Caucus, reflecting mounting despair.President Barack Obama and Texas Governor Greg Abbott asked for the public's prayers. In a letter posted online on Friday, Abbott said "every life matters" and urged Texans to come together."In the end," he wrote, "evil always fails."Johnson, 25, had amassed a personal arsenal at his home in the Dallas suburb of Mesquite, including bomb-making materials, rifles, ammunition and a journal of combat tactics, authorities said on Friday.He followed black militant groups on social media, including one that posted a message on Wednesday encouraging violence against police.Johnson was a private first class with a specialty in carpentry and masonry. He served in the Army Reserve for six years starting in 2009 and did one tour in Afghanistan from November 2013 to July 2014, the military said.When Johnson was accused of sexual harassment by a female soldier in Afghanistan, he was sent back to the US with the recommendation he receive an "other than honorable" discharge, but he later got an honourable discharge, said Bradford Glendening, a military lawyer.Parking garageIn addition to the five slain officers, seven officers and two civilians were wounded.The episode began on Thursday evening while hundreds of people were gathered to protest the police killings of two more black men: Philando Castile, who was fatally shot near St Paul, Minnesota and Alton Sterling, who was shot in Louisiana after being pinned to the pavement by two white officers.After shooting at the Dallas officers, Johnson tried to take refuge in a parking garage and exchanged gunfire with police, Police Chief David Brown said.The suspect described his motive during negotiations and said he acted alone and was not affiliated with any groups, Brown said. Police initially suspected more than one shooter.He said he wanted to exterminate whites, "especially white officers," officials said.In Washington, the nation's top law enforcement official, Attorney General Loretta Lynch, called for calm, saying the recent violence can't be allowed to "precipitate a new normal."Lynch said protesters concerned about killings by police should not be discouraged "by those who use your lawful actions as a cover for their heinous violence."Passing carsThe other attacks on police included a Georgia man who authorities said called 911 to report a break-in, then ambushed the officer who came to investigate. That sparked a shootout in which both the officer and suspect were wounded but expected to survive.In suburban St Louis, a motorist shot an officer at least once as the officer walked back to his car during a traffic stop, police said. The officer was hospitalised in critical condition.A Tennessee man accused of shooting indiscriminately at passing cars and police on a highway told investigators he was angry about police violence against African-Americans, authorities said.Elsewhere in Texas, police shot and killed a man early on Saturday after he was spotted standing in a Houston road with a revolver. Authorities said officers told the man to put down the weapon, but he instead pointed it in the air, then at the police.