Berlin and Hamburg are being hit by a worsening epidemic of random "car-torching" attacks in which more than 600 privately owned vehicles have been set ablaze this year in apparently motiveless attacks, which are baffling police, politicians and the public.
In Berlin, which faces a city government election next month, the problem has prompted Chancellor Angela Merkel's conservative CDU party to launch a poster campaign with disturbing images of burnt-out cars and the slogan "Berlin must not turn into London".
On Monday, a dozen cars parked in the streets of the normally quiet and respectable west Berlin boroughs of Charlottenburg and Rudow were set ablaze and burnt out in late-night arson attacks. Three more cars followed on Tuesday.
Fears about buildings catching fire have caused Ms Merkel to warn the mystery arsonists that they were "gambling cold-bloodedly with people's lives". Yet in nearly all the cases, the perpetrators have managed to escape undetected.
In most attacks, cars are torched during the small hours with the kind of firelighters normally used for garden barbecues. They are left to burn slowly under a car's tyre, leaving the culprit plenty of time to get away.
The German capital has seen 364 arson attacks on cars this year alone, while the port city of Hamburg is not far behind. In both cities an estimated 2,000 cars have been set alight and destroyed over the past seven years, yet police now admit that the problem has suddenly dramatically worsened.
Previous car-torching incidents have been confined to run-down east Berlin. In most cases police have attributed the attacks to radical left-wing and anarchist groups violently opposed to such inner-city areas being developed by "big business".
However, police and politicians admit that they are largely baffled by the latest trend. "We assume that around half of the arsonists are politically motivated, but we think there are more lone perpetrators who are not organised by groups," Ehrhart Körting, Berlin's Interior minister, said.
In their latest attempt to catch the mystery arsonists, police have deployed helicopters with heat-seeking cameras. In Berlin police have also drafted in some 350 extra officers from outside the city to help. Yet so far their efforts have proven fruitless.