THERE were five teenagers in the car rolling quietly through the muggy night an hour before dawn in central New Orleans. By the time the police caught up with them last weekend, they were all dead — riddled with bullets in the worst outbreak of gun violence since Hurricane Katrina devastated the city last year.

An apparent ambush by unknown gunmen in an area still recovering from Katrina’s flooding has delivered a sharp jolt to what many criminologists are describing as complacency about violent crime.

As more than 100 reservist military police and 60 state troopers were sent to reinforce the police last week, New Orleans was emerging as a blood-spattered symbol of an alarming trend in US criminality.

The city that briefly enjoyed a zero murder rate in the wake of the evacuations forced by Katrina is on the way to regaining its status as America’s most dangerous city, even though its population has halved.

After 54 murders this year, New Orleans has become one of the deadliest examples of a rising murder rate that is defying the trends of the past decade and fuelling concern about a crime wave that appears to be spreading to smaller cities and states with little experience of serious gun violence.

From Tulsa, Oklahoma, to Milwaukee, Wisconsin, cities that rarely make a dent on the FBI’s annual statistical surveys are recording sharp jumps in violent crime.

Researchers are re-examining the factors that may be influencing crime, among them the migration of middle-class populations to the “exurbs”, or distant suburbs, often with weak police forces unable to handle the influx and little experience of urban crime.

Experts say they do not fully understand why murders shot up 10% or more last year in cities such as Buffalo, in New York state, Memphis, in Tennessee, and Little Rock, in Arkansas. But they believe that the distractions of fighting terrorism and a possible migration of criminals chased out of the bigger cities may have contributed to a jump of 5% in America’s overall murder rate last year — the biggest rise in a single year since 1991.

“These are not the bad old days yet,” said Professor Jack Levin, a criminologist at Northeastern University in Boston. “But I don’t think it’s a blip on the radar screen either. I think it’s the beginning of a trend and we could see a whole new crime wave.”

For more than a decade American police chiefs have been “sticking their chests out” while claiming credit for falling crime rates, noted Professor Peter Scharf, a criminologist at the University of New Orleans. US crime-fighting techniques such as zero tolerance, computerised analysis and focused deployment of resources have been used around the globe.

Some cities continue to record significant falls in crime — notably New York, where the number of murders dropped again last year to 539 compared with 2,245 in 1990 — and the trend has been reinforced by long prison sentences that keep potential killers off the streets.

Yet police in San Antonio, Texas, have been scratching their heads over the causes of a dramatic 70% increase in the murder rate this year. Traditionally peaceful states such as New Hampshire, Iowa and Kansas are also experiencing violence unknown a decade ago.

In some cities, rises in the murder rate have been linked to the arrival of Katrina evacuees. Yet FBI officials shied away from blaming any single factor for increases elsewhere.

“Our large cities seem to be doing something right,” said Regina Schofield, an assistant attorney-general in the justice department. “We need to find out what’s going on in the smaller cities.”

One official acknowledged that some communities were “less prepared” to deal with the activities of criminal gangs displaced from bigger cities. Richard Hertling of the justice department said he had heard of experienced criminals moving out of California, which has a “three strikes and you’re out” law for repeat offenders, and setting up operations in Nevada, Oregon and Arizona.

Officials are also concerned about the release of thousands of prisoners arrested during the 1980s war on drugs who have now served their sentences.

Several criminologists argue that US officials have been distracted by terrorism since the September 11 attacks and that the government is complacent about ordinary crime. “We’ve been resting on our laurels,” said Levin.

Researchers are re-examining potential crime-breeding factors, from the role of abortions in poor communities — theoretically preventing criminals being born — to the spread of more lethal weapons.

“In West Side Story [the musical] they fought with switchblades,” said Scharf. “Now you might as well be in Falluja. When you give kids automatic weapons nothing good is going to happen.”

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