Micah Johnson was a Black Power extremist who had a white stepmother, a soldier who served his country in Afghanistan then shot dead fellow veterans when he came home.

He worked with special needs children but hoarded a terrifying arsenal of guns in his mother's house. To the world he presented a "chilled" persona, but on the subject of race he would erupt in rage.

Before he opened fire in Dallas on Thursday night, murdering five policemen and injuring seven more, there had been growing signs that Johnson was dangerously unhinged.

He had been dismissed from the Army in disgrace after a female soldier accused him of sexual harassment and said he needed "mental help," and a neighbour had spotted him conducting military exercises in his mother's garden.

There was also lingering resentment from an incident when he was was stopped and questioned by police for two minutes while sitting in his parked car.