They normally strike at some time between 2 am and 5 am.

A loud boom, the pressure wave smashed window panes to pieces and then the sound of a motor screaming in the night.

Sometimes only minutes go by until the next detonation. Sometimes several nights go past and nothing happens.

In North Rhine-Westphalia (NRW) police are chasing several gangs who blow up ATMs to get at the money inside.

“There has already been several million euros worth of damage,” confirmed a spokesperson for the state police.

Investigators believe the criminals often come from the Netherlands, since the attacks occur most often in the border region between the two countries.

How much money the thieves have made off with so far is a closely guarded secret. Police and banks don't want to give criminals further incentive. But for those carrying out the crimes the amounts they've stolen so far seem to be incentive enough.

That a special police squad has been tasked to track them down has also had little deterrent effect – if anything the incidents have increased.

In 2015 there have so far been 45 cases in NRW. With 63 cases nationwide, it's clear the offence is focused on Germany's most populous state.

Five minute job

“The criminals are never at the crime scene for longer than five minutes,“ said a police spokesperson. Within this time they have prepared the cash machine with a combustible gas mixture, trigger the explosion and taken their loot.

Given the power of the blasts it's “pure luck“ that no one has yet been injured, claimed the spokesperson.

Eight cases have so far been cleared up – police tracked them down to an eastern European gang – two of whose members were arrested, while the other three are on the run.

Another ring were almost caught in September when police tried to search an Audi. But the luxury car sped off at 250 km/h, leaving the squad cars in its wake.

At least 21 police cars and a helicopter were involved in the chase.

But after zooming down seven motorways the criminals finally reached sanctuary in the Netherlands. And as if they wanted to make a point of making police look ridiculous, the gang even stopped to refuel their car on the way.

Some politicians in the west German state are furious, accusing the interior ministry of falling in its duty to protect.

But NRW's interior minister Ralf Jäger laid the blame with the banks.

“We need to reduce the incentive for the gangs,“ he said. “Why do they come to us? Because in Belgium and the Netherlands the ATMs are stocked with paint pellets. I hope that the banks here will copy them.“

Indeed, similar crime waves were stopped in their tracks in the Netherlands and Belgium when paint pellets were loaded into the cash points, rendering the notes unusable in the event of an explosion.

Some banks in NRW have already started locking up the lobbies of their banks overnight to protect the ATMs.

According to the boss of Wincor-Nixdorf – an ATM maker – the attacks can be stopped, it is just a question of how much money one is prepared to invest.