German police are investigating whether a string of arson attacks that have disabled parts of the rail network across the country could be part of a coordinated scheme with a political motive.

Overnight fires at signal boxes and cable ducts caused huge delays on rail routes in and out of Berlin, Hamburg, Cologne, Dortmund, Leipzig, Bremen and Bad Bevensen.

The interior ministry spokesman, Tobias Plate, said there had been 13 cable fires caused by “unconventional explosive and incendiary devices”.

Though temperatures reached 30C (86F) in parts of Germany on Sunday, police discounted the possibility that the fires had been caused by the hot weather, pointing out that one cable fire in Berlin had started at 3.25am on Monday.

A spokesperson confirmed that German federal police were investigating the possibility of a political motive behind the attacks.

In a statement uploaded on the leftwing website Indymedia on Monday morning, a group called Shut Down G20 claimed responsibility for the attacks, describing it as “an assault on capitalism’s central nervous system”.

Activists from a range of political causes are planning large-scale protests at the G20 summit in Hamburg from 7-8 July, which will be attended by political leaders including Donald Trump, Vladimir Putin and Recep Tayyip Erdoğan.

A police spokesperson said the authenticity of the statement was being analysed. In the past, Indymedia has been used to disseminate fake statements claiming incidents such as the bombing attack on Borussia Dortmund’s team bus.