By Ludwig Burger

FRANKFURT, Sept 4 (Reuters) - An attempt has been made to short-circuit an electricity grid transformer south of Duesseldorf in western Germany, according to ‌police, the newspaper Bild reported on Friday, heightening a state of alert ‌after two suspected sabotage attempts this week.

Cologne police, in the same region, told Reuters they had knowledge of a ​letter claiming responsibility for the incident in Dormagen as well as vandalism at a substation in nearby Bergheim that took 4,200 megawatts in generating capacity offline on Tuesday. They declined to give further details while the investigation was under way.

Bild reported that a man had claimed responsibility ‌for both incidents in a ⁠handwritten letter, in which he also revealed his identity, but it was unclear whether he was the perpetrator.

Germany has been the focus ⁠of European fears about the vulnerability of critical infrastructure this week.

Allies rallied round after Germany blamed a drone attack in August at Leipzig/Halle airport on Moscow as part of hybrid warfare ​meant to ​intimidate and divide countries that have supported Ukraine ​since Russia's full-scale invasion in 2022. ‌Russia has dismissed the allegations as baseless.

On Sunday, a closely watched election takes place in the eastern state of Saxony-Anhalt, where the far-right, Russia-friendly Alternative for Germany (AfD) holds a commanding lead in surveys.

Germany's Interior Ministry said it was "no coincidence" that such "hybrid attacks" were happening on the eve of the election.

A spokesperson for Chancellor Friedrich Merz said the number ‌of attacks that had been foiled or been ​unsuccessful were "a sign of how our security measures work – ​and that they do work".

In the ​other sabotage incident this week, devices carrying conductive material damaged power lines ‌at one of the most critical high-voltage ​nodes in the eastern ​state of Brandenburg.

Separately on Friday, police in the town of Ganderkesee near Bremen found the door to a control enclosure at a transformer station open at ​3 a.m., and searched the ‌area with a helicopter and dogs.

The facility was undamaged and nobody was ​found but an investigation was continuing, police said.

(Reporting by Ludwig Burger; editing by ​Kirsti Knolle and Editing by Kevin Liffey)