NEW YORK (NYTIMES) - The stolen cars reappeared on the streets of Hartford, Connecticut, so often over the past year that police would sometimes find five in a day.

But unlike the hot-wiring heydays of the 1980s and 1990s, when chop shops thrived, most of the recovered cars were unharmed.

After years of declines, car thefts appear to be surging in cities and suburbs all over the country. The spree, which has been exacerbated by the pandemic, does not appear to be the work of sophisticated crime rings, police say.

Instead, this new wave of car thefts seems to stem from a combination of simple carelessness and the same technological advancement that once made stealing cars nearly impossible: the key fob.

The broad adoption of keyless ignitions that began in the late 1990s ushered in a dark era for car thieves.

New cars had engine immobilisers that only a microchip in the key fob could unlock, and vehicle thefts quickly plummeted. From a high of 1.7 million a year in 1991, thefts had dropped more than 50 per cent in recent years, according to data compiled by the FBI.

Technology, it seemed, had largely solved the problem of stolen vehicles. Until people started leaving their fobs sitting in their cup holders.

Now, police say forgotten fobs and keyless technology have contributed to soaring stolen car cases that do not look much like the crimes that plagued cities three decades ago.

In Hartford, police have traced the surge to teens joyriding in from the suburbs. In Los Angeles, stolen cars reappear so frequently that police believe thieves are using them like Ubers.

And in New York City, a related but different problem has emerged as more drivers leave their cars running to make pit stops and deliveries during the pandemic, making their cars easy targets for thieves who can simply drive away, even without a fob.

The issue so concerned the New York City Police Department that it recently made a video featuring a man whose car was stolen - with his French bulldog inside - to warn New Yorkers of the risk.

"This is a very stupid problem to have," a Hartford Police Department official said to reporters last month, on a day when five stolen cars were recovered in the city, and 12 people - about half are teenagers - were arrested.

"The technology that was created specifically to eliminate car thefts, such as key fob technology, is now being used against us."

The situation has left law enforcement struggling to keep up with a deluge of car thefts; in some places, the endless caseload has threatened to overburden smaller police departments.

But unlike the crimes of yesteryear, when stolen cars would eventually turn up stripped for parts, police say most today are abandoned undamaged. And with laborious tracing work, police say they are able to return many stolen cars to their owners.

The pandemic has made the problem worse, said Deputy Inspector Jessica E. Corey of the New York City Police Department's Crime Prevention Division.

With the increase in deliveries as people try to stay home, many victims are delivery drivers making drop-offs, she said, stressing that in many of the city's cases, people have also left cars running with traditional keys.

In New York City, 6,858 vehicles were stolen last year, up from 3,988 the year before. Of those taken last year, more than 3,450 were stolen while they were running. The year before, 1,634 were stolen while running. (The department does not specify whether electronic or traditional keys are used.)

A snapshot of a single New York City day is revealing: On Dec 5 alone, 11 vehicles were stolen while left running, and another six were taken with keys or fobs inside, according to the police department.

There are many ways to leave a car vulnerable: Some drivers forget a key fob inside. Others take it, but leave the car on, allowing the vehicle to be driven away, though not restarted later. Some cars can be started if the key is just nearby.

And in a smaller number of cases, criminals have used technology to reprogram keyless cars.