(AFP) The Dallas ambush by a black gunman targeting white police is the latest spasm of fury that reflects what experts see as a rising tide of extremism and intolerance on the fringes of American society.
Fueling the country's polarization are racial tension, anger with establishment politicians and economic inequality, says the Southern Poverty Law Center, which monitors racist and hate groups in the United States.
Micah Johnson, the 25-year-old army reservist who killed five police in Thursday's sniper attack, sympathized with black militant groups, the center said Friday.
Johnson told police negotiators before he was killed that he was acting out of anger at the deaths this week of two black men at the hands of police in Minnesota and Louisiana, and said that he wanted to kill white people, particularly white police officers.
The SPLC has written about what it calls black separatists; it says they "typically oppose integration and racial intermarriage, and they want separate institutions - or even a separate nation - for black people in America."
These groups are "generally very small, but they are very anti-Semitic, very anti-white and also very anti-gay," said Mark Potok, a SPLC analyst who has written a study on the rise in hate groups over the past year.
Black separatists are very different from groups like Black Lives Matter, which arose in response to the many cases in America of recent years of unarmed blacks dying at the hands of police officers, often white ones.
But while not linked to the Black Lives Matter movement, they have benefited from its popularity, he told AFP.
"All of the anger out there about police violence against black people has helped the extremist black groups grow," said Potok.
For this reason, Black Lives Matter was quick to distance itself from the Dallas atrocity on its Facebook page Friday.
"Yesterday's attack was the result of the actions of a lone gunman," it wrote. "To assign the actions of one person to an entire movement is dangerous and irresponsible."
On Facebook, Johnson followed the New Black Panther Party, the Nation of Islam and Black Riders Liberation. All of these are listed as hate groups by the Southern Poverty Law Center.
The New Black Panther Party blames Jews for the September 11, 2001 terror attacks, and argues that whites are planning a genocide against all non-white people, according to the SPLC.
The Nation of Islam is also known for virulent anti-Semitism and hatred toward whites.
Nerves on edge
The Dallas shooting gives pause to consider the rise of intolerant movements - of many forms - in the United States.
Groups identified online as militia movements, white supremacists, Islamist radicals, neo-Nazis and other kinds of extremists increased from 784 to 892 between 2014 and 2015, a 14 percent rise, the SPLC said.
The figure has doubled since 1999, when the SPLC identified 457 hate groups across the country.
The majority are racist groups - with Ku Klux Klan-linked and black separatist movements accounting respectively for 21 percent and 20 percent of all hate groups.
Groups that identify as being affiliated with the KKK have risen from 72 in 2014 to 190 in 2015. Last year, a young white supremacist killed nine black worshippers at a church in Charleston, South Carolina.
At the other end of the spectrum, black separatist groups rose in number from 113 to 180 last year amid anger over the deaths of blacks at the hands of police officers.
President Barack Obama, referring to the deaths this week in Minnesota and Louisiana, said they were not isolated incidents.
"They are symptomatic of the broader challenges within our criminal justice system, the racial disparities that appear across the system year after year," Obama said.
Examples abound of how nerves are on edge all over America.
On Thursday, with the country already tense over the twin shootings of African-Americans, whose dying moments were captured in now-viral video footage, the body of a black youth appeared hanging from a tree in Atlanta.
Police ruled it a suicide. But local people reacted with wariness to a scene reminiscent of KKK lynchings decades ago.
For Scott Simpson, spokesman for the Leadership Conference on Civil and Human Rights, the shrill tone of US political discourse has a lot to answer for.
A White House campaign notable for the anti-immigrant and racist rhetoric employed by the Republican frontrunner, Donald Trump, "creates an environment where extremism becomes more tolerable," he said.
"When hateful rhetoric is in the mainstream, it gives license to people with extreme points of view who may or may not want to discuss it in public otherwise," said Simpson. "It gives license to people to target communities."