When politicians – especially Donald Trump – ratchet up the rhetoric about perils from Mexico along the southern border of the USA, citizens of El Paso react with indignation – also bewilderment, by recourse to a stark truth: they are much safer than most other Americans.

Until now.

In April last year, El Paso again made it into the “top 10 safest cities in America’, complied by the security company Safewise. So did San Diego, also on the border.

This weekend, however, El Paso did not feel like one of the safest places in America. That marvellous city has lost that title, not to the violence Trump forever invokes – by Hispanic “rapists”, “drug-dealers” or “criminals” from across the frontier – but to the bloodletting of a white man bent on killing Hispanic familieswhile they were out shopping.

The irony could not be more cruelly heartbreaking.

Trump said in his State of the Union address last February that “the border city of El Paso used to have extremely high rates of violent crime … one of our nation’s most dangerous cities” – until a border fence was built. He was greeted with a barrage of truths: first that, according to FBI data, El Paso’s rates of homicide and violent crime have long been very significantly lower than in cities of comparable size (population 683,577 last count).

Second, what violent crime there is, having plunged since a peak during the mid-1990s, began to rise slightly after the passing of the Border Fence Act by the George W Bush administration in 2006 (by 9.6% to 2011).

The assumptions that propel Trump’s bile and the alleged assassin’s “manifesto” currently under investigation, is that the threat to “security” along the border comes from immigrants, migrants and cartels.

But none of these categories are – or ever have been – a threat to border communities. Most folk living along the frontier in majority-Hispanic communities like El Paso are law-abiding, hard-working, tax-paying, decent and honest US citizens and green-card holders, who crisscross the frontier every day as part of their family, social and professional life. Domestic finances are transnational.

Migrants have been forced by brutal circumstance to seek refuge in border towns, dramatically so in El Paso. But there is no evidence whatsoever that any have contributed disproportionately to crime.

Narco-trafficking cartels working from the metropolis of Ciudad Juárez, opposite El Paso, have no interest in drawing heat along the border. Their business is across America, and that is where, once clear of the line, their couriers go.

El Paso is left well alone to go about its own, peaceful business – until someone arrives in town after having allegedly written that: “Hispanics will take control of the local and state government of my beloved Texas … Our European comrades don’t have the gun rights needed to repel the millions of invaders.” That’s when mass murder came to El Paso, not before.

I wrote a book a decade ago, entitled Amexica for obvious reasons: it attempted to explore a richly unique territory, during a time of violence on the Mexican side, through which the frontier passed. A place that belonged to both countries, yet neither; an addictive borderland with a border culture, to which I return often.

El Paso and Juárez face and blend into one another across the Rio Grande

And yet it is dichotomous: the frontier both porous and harsh. People die trying to cross it, yet – and this people forget – one million did and still do so every day, legally. The clunk-click of turnstiles is the soundtrack to this terrain; hundreds of billions of dollars-worth of goods traverse the line in both directions every year.

El Paso and Juárez face and blend into one another across the Rio Grande. Of all the binational metropolises along the 1,934 miles of border, they are its fulcrum. Time spent in either is a joy usually, because of things we never read about: the ready smile, the unnecessary chat, big hearts.

At one level, like the other twin cities – Tijuana-San Diego; Calexico-Mexicali, Nogales-Nogales, Del Rio-Acuña, Eagle Pass-Piedras Negras, Laredo-Nuevo Laredo, McAllen-Reynosa and Brownsville-Matamoros – they share a main street, with a border drawn across it.

In his book, The Line Becomes a River, former border patrol agent Francisco Cantú observes how the lights of El Paso and Juárez “reached across the border to form a single, throbbing metropolis”.

Scores of times, before and since writing Amexica, I’ve sat on the balcony outside my favourite hotel room in the world – 252 at La Quinta, El Paso - staring at those lights of charismatic, mighty Juárez, from the safety of El Paso, ready to dive in next morning.

Cantú goes on to say, rather dramatically: “to live in the city of El Paso was to hover at the edge of a crushing cruelty, to safely fill the lungs with air steeped in horror”.

But the horror never crossed the line. It arrived from the opposite, northerly, direction.

Ed Vulliamy was the Observer’s US correspondent between 1994 and 2003, and is the author of Amexica: War Along the Borderline (FSG/Picador)