AMERICA can only hold its breath. It rocks on the precipice of crisis amid unbearable tensions following the slaughter of five police officers in a gun attack at a protest rally in Dallas, itself organised after another shooting of two black men by police.

As US President Barack Obama looks to cut short his trip to Europe and head home in an attempt to heal a nation, no-one quite knows what, or where, something will happen next.

On every street, in every State, fear and mistrust lingers.

The deadliest day for US law enforcement officers since the 9/11 attacks in 2001 comes as relations between America's black community and a police family plunged into mourning sink to a depressingly all time low.

But in Dallas itself, signs of hope. Of a community rebuilding, with some public displays of unity. People of all colours shaking hands and hugging grief stricken officers, a bond of unity previously eroded by accusations of trigger happy police culture all too ready to use lethal force.

Buildings were bathed in blue light, an architectural tribute to the fallen officers whose uniforms turned them into targets.

Two squad cars parked outside the Dallas police HQ, completely festooned in floral tributes, an explosion of colour in stark defiance of the force's darkest day.

And in a dramatic intervention yesterday, US Vice-President Joe Biden calling for a country stricken by grief to come together.

He said that slaying of the five police officers in Dallas has "touched the soul of the nation".

Biden said people had a duty to stand up against injustice, but that they also needed to support police, as he referenced the killings of the two black men Philando Castile in Minnesota and Alton Sterling in Louisiana.

"When an assassin's bullet targeted the police force in Dallas, it touched the soul of the nation," he said.

"Those killed and wounded were protecting the safety of those who were peacefully protesting against racial injustices in the criminal justice system", he added in an impassioned plea for calm while filling in for Barack Obama in the weekly Presidential radio address,

But even as the nation reached for answers, it emerged Micah Johnson, the man who killed the five police officers in the Dallas gun attack, had his own demons to fight.

He had been sent home from Afghanistan after being accused of sexually harassing a female and was described as a loner.

It appeared he followed black militant groups on social media despite former friends claiming the 25-year-old black man not previously having an interest in politics.

But his Facebook page suggested otherwise.

He had "liked" black militant groups including the African American Defence League and the New Black Panther Party, which was founded in Dallas.

A photo of Johnson showed him wearing a dashiki and raising his fist over the words Black Power, and his cover shot carried the red, black and green Pan-African flag.

Activists with Black Lives Matter, whose peaceful march police were guarding as he opened fire, repudiated the shootings, and it was not immediately clear if Johnson had any connection to the movement, which has disavowed violence.

But one of the groups Johnson "liked" on Facebook, the African American Defence League, had posted a message earlier in the week encouraging violence against police in response to the killing in Louisiana.

"The Pig has shot and killed Alton Sterling in Baton Rouge, Louisiana! You and I know what we must do and I don't mean marching, making a lot of noise, or attending conventions. We must 'Rally The Troops!' It is time to visit Louisiana and hold a barbeque."

The message was attributed to Dr Mauricelm-Lei Millere, a leader in the organisation.

Dallas police chief David O. Brown said Johnson, who was eventually also killed during the assault in Dallas, told negotiators he was acting alone and was unaffiliated with any group but cited the fatal shootings of Castile and Sterling as a source of his anger.

"The suspect said he was upset with white people and wanted to kill white people, especially white officers," Brown said.

Background checks by the military have revealed Johnson served in the army reserve for six as a private first class with a speciality in carpentry and masonry starting in 2009.

In May 2014, six months into his Afghanistan tour, he was accused of sexual harassment by a female soldier.

The army sent him home, recommending an "other than honourable discharge", said Bradford Glendening, the military lawyer who represented him.

Glendening said Johnson was scheduled to be removed from the army in September 2014 because of the incident, but instead got an honourable discharge months later.

"Someone really screwed up," Glendening said. "But to my client's benefit."

When authorities searched Johnson's home they found bomb-making materials, ballistic vests, rifles, ammunition and a personal journal of combat tactics.

The recriminations to follow come as President Obama looks to abandon a European visit and travel to the site of the killing of the five police officers.

He spoke of a renewed new effort to ease tensions between law enforcement and African Americans that have been at breaking point for almost two years.

Obama said all Americans should be "deeply troubled" by the shootings in Baton Rouge, Louisiana, and St Paul, Minnesota.

And for its part, well-know rap artists rallied to the call.

On his Facebook page Griffin, a member influential hip hop group of Public Enemy known for its politically charged lyrics - said he does "not advocate killing Cops".

He was followed by rappers Snoop Dog and The Game also called for more "dialogue" between police and campaigners in the wake of all the killings.

But in a sign of the level of tensions that remain, former Congressman and Tea Party activist Joe Walsh Tweeted out that there was now a state of war in America.

He cautioned President Obama to “watch out" adding that "Real America is coming after you".

Angry protests first broke out over the death of 32-year-old Castile.

He worked as a cafe supervisor at a local school when he was shot dead in front of his girlfriend and daughter after his car was pulled over by a police officer .

His girlfriend Lavish Reynolds - who streamed the aftermath of the incident live on Facebook - says he was killed "for no apparent reason".

In the video, which went viral, she describes being stopped for a "busted tail light" and says Castile had told the officer he was carrying a gun for which he was licensed.

So too Sterling's death in Baton Rouge, filmed by a passer-by who was just yards away when two white police officers tried to arrest the 37-year-old.

Sterling, a father of five who had been detained a number of times before, had been selling CDs outside a convenience store.

Police say they had been sent to the store after an anonymous caller told them Sterling had threatened someone with a gun.

In the mobile phone footage, the two officers are seen pinning Sterling to the ground, and someone yells "He's got a gun! Gun!" before shots are fired.

As the unrest over the shooting raged as acclaimed US filmmaker and campaigner Michael Moore highlighted claims on social media that US police were killing more black citizens today than were lynched at the height of American segregation laws.

In a posting on Twitter Moore stated: "More black ppl killed by police in 2015 than were lynched in the worst year of Jim Crow segregation laws (1890-1965)".

Moore highlighted data from the Quartz news website that said a total of 2,911 black Americans were lynched between 1890 and 1965, when the so-called Jim Crow laws were enforced. Beginning in the 1890s, these racist laws segregated black Americans in several states until about 1965.

On an average, 39 black people were lynched per year under Jim Crow. In 1892, the worst year, 161 black Americans were lynched.

More than a century later, the numbers have hardly improved. In 2015, 258 black people were killed by US police, representing over 26% of deaths, the Quartz website stated.

In a Tweet, Moore added: "We can also control police killing black men & other citizens. We need a national standard of police training & screening. Most cops r good."

Moore added: "Let's hope we use this moment. Racism ingrained in our system must be eradicated.

"African Americans live in fear of police - this must end."