- Dallas police shootings: live coverage
- Dallas police officers shot dead
- US police shooting: Philando Castile shot dead
- Alton Sterling shot dead in Baton Rouge
A nation already struck with fear after a string of mass shootings, police killings, riots and terrorist attacks over recent years has again watched horror spread and blood spill across its streets.
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Dallas Shooting: as it happened
Dallas shootings caught on Twitter
Twitter user captures the moment heavy gunfire rings out in Dallas where 12 people have been shot, four killed.
This time the city was Dallas, Texas, and the targets were police officers, 11 of whom were gunned down, five of whom died, by one or more snipers during what had been a peaceful protest on Thursday night against two police killings of African American men.
The motive for the Dallas shooting is as yet unclear. It may revenge against police, the protest may have been hijacked for some other reason.
What compounds the horror for America though is the real risk of violence metastasising across its cities.
Recent riots have already scarred Baltimore in Maryland and Ferguson in Missouri, where community fury at police killings of African-Americans boiled into kinetic rage.
Two police officers were murdered as they sat in the car in New York in 2014, retribution, the gunman claimed before he took his own life, for police killings.
And America does not have to look far into its history to see how far and fast this sort of violence can spread. In 1992, 55 people died in Los Angeles in riots sparked when police were acquitted for the assault of Rodney King.
Many cities in America still bare the scars of the race riots of 1967 when whole inner-city neighbourhoods were torched. Indeed some the buildings that burned last year in Baltimore adjoined blocks empty since '67.
And then there is issue of guns, which serve only to compound the violence in America. In a late-night press conference, the Dallas police chief said his officers came under rifle fire from more than one shooter. Time and again American mass killers demonstrate the efficacy of the military-style semi-automatic rifles that are so freely available across the country.
It is guns too that have spurred police to use lethal force so readily. In their daily duties American police rightly fear that those they deal with may be armed.
This appears to have been a contributing factor in the death of Philando Castile, who was shot dead by a Minnesota police officer this week in one of the killings that prompted the Dallas protest. Castille was pulled over for a busted brake light as he drove with his fiance and her four year old daughter. He told the officer at his window he was licensed to carry a gun and - according to his fiance - reached for his wallet when he shot four times.
The spread of guns might even have already complicated the Dallas investigation.
Texas is an "open carry" state, meaning that it is perfectly legal for citizens to carry military-style rifles with them as they go about their business.
One African-American man was pictured carrying a rifle during the protest. Later, according to social media accounts, he turned himself in with his brother after discovering that he had become a "person of interest" for the shootings.
It is the sort of complication unimaginable in most peaceful nations that is becoming another mark of public life in America.