Given a choice between their worldview and the facts, it's always interesting how many people toss the facts. Right now, the United States is plagued by an army of "birthers" who claim that because Barack Obama was not really born in America, he's not legitimately president. Their evidence is non-existent, their arguments loopy, but people who find our non-white president unacceptable would rather scour the Hawaiian medical records system and invent bizarre theories than face their own internal turmoil. Or racism.

What people were willing to believe about Hurricane Katrina in New Orleans four years ago is a more serious matter. Of racism. And cliche. The story, as the mainstream media presented it at the time, was about marauding hordes of looters, rapists and murderers swarming through the streets. The descriptions were pretty clearly focused on African-Americans, the great majority left behind in the evacuation of the city (which was then two-thirds black anyway).

There were supposed to be a lot of murder victims and murderers in the Superdome, the sports stadium the city opened up as a refuge of last resort. The rumours were believed so fervently that they were used to turn New Orleans into a prison city, with supplies and would-be rescuers prevented from entering and the victims prevented from evacuating. The belief that a Hobbesian war of all-against-all had broken loose justified treating the place as a crime zone or even a hostile country rather than a place in which grandmothers and toddlers were stranded in hideous conditions, desperately in need of food, water, shelter and medical attention.

Louisiana's governor at the time, Kathleen Blanco, announced as she dispatched National Guard troops: "I have one message for these hoodlums: these troops know how to shoot and kill, and they are more than willing to do so if necessary, and I expect they will." She and the city's mayor had called off the rescue efforts to focus on protecting private property – with lethal force if necessary. The sheriff of the suburb across the Crescent City Connection bridge from downtown New Orleans turned back stranded tourists and locals at gunpoint. "As we approached the bridge," wrote two stranded paramedics, "armed Gretna sheriffs formed a line across the foot of the bridge. Before we were close enough to speak, they began firing their weapons over our heads."

Katrina was a fairly terrible natural disaster. But it turned into a horrific social catastrophe because of the response of the people in power, spurred on by their willingness to believe a hysterical, rumour-mongering media. (Journalists on the ground were often fiercely empathic and right on the mark, but those at a remove were all too willing to believe the usual tsunami of cliches about disaster and human nature.)

The story that few can wrap their minds around is that ordinary people mostly behaved well – there were six bodies in the Superdome, including four natural deaths and a suicide, not the hundreds that the federal government expected when it sent massive refrigerator trucks to collect the corpses. On the other hand, people in power behaved appallingly, panicking, spreading rumours, and themselves showing an eagerness to kill and a pathological lack of empathy.

Amusingly, the New Orleans Police Department stripped a Cadillac dealership of its cars, some of which were found as far away as Texas. Less amusingly, they shot a couple of unarmed – and, of course, black – family groups on the Danziger Bridge shortly after the storm in the only such incident to receive much press coverage. A middle-aged mother had her forearm blown off; a mentally disabled 40-year-old on his way to his brother's dental office was shot five times in the back and died, and a teenager was also killed.

Truth, the first casualty of war, is pretty imperilled in disasters, too. One group of suburban white men who believed the rumours or just anticipated that in the absence of authority we all become monsters became monsters themselves, even as they fantasised they were preserving order. These men in Algiers Point across the river from the city of New Orleans gathered an arsenal and launched their own little murder spree, killing several black men and injuring and threatening others.

They were the real rampaging gangs, and they were not shy about what they did – they boasted of it to videographers and have talked openly about it since. And with confidence, since there have to date been no legal repercussions. They claimed to be defending their property and their neighbourhood, but their most vocal surviving victim, Donnell Herrington, was an armoured truck driver trying to evacuate after he had stayed behind to take care of his grandparents. Herrington, who rescued those grandparents and dozens of neighbours by boat from their flooded apartment complex, then tried to find an evacuation point in Algiers for himself, and was shot twice at close range with a shotgun and nearly bled to death before neighbours got him to the hospital. The vigilantes shot him because he was black, and because they could get away with it, and because they were inflamed by the news accounts.

The story was not hard to find, and I picked up a lot of pieces of it while doing research for a book on disaster and civil society. Though New Orleans was overrun by national and international journalists, no one would touch it until I enlisted the brilliant investigative journalist AC Thompson. Despite his cover story in the Nation that included admissions of murder, many still deny that the killings took place. Given a choice between their worldview and the facts, some choose the worldview.

Most people behave beautifully in disasters (and most Americans, incidentally, believe Obama was born in this country). The majority in Katrina took care of each other, went to great lengths to rescue each other – including the "cajun navy" of white guys with boats who entered the flooded city the day after the levees broke – and were generally humane and resourceful. A minority that included the most powerful believed they were preventing barbarism while they embodied it.

We are entering an era of heightened disaster, thanks to climate change. Being prepared for disaster will mean being prepared to sift truth from rumour, and being prepared to adjust our worldview. There is some incredible ugliness to the truth about Katrina. But, four years on, the lies hide more beauty, and hide where our dangers and our salvation may lie in times of crisis.

Rebecca Solnit's new book is A Paradise Built in Hell: The Extraordinary Communities That Arise in Disaster