BERLIN — An extreme left-wing group has claimed responsibility for an arson attack that caused a blackout affecting about 45,000 households and more than 2,000 businesses in Berlin over the weekend.

“This isn’t just arson or sabotage. It’s terrorism,” Berlin’s Mayor Kai Wegner said Sunday of the attack, which burned through a cable connected to one of the city’s largest gas-fired power plants.

Members of the so-called Vulkan Group, known for similar attacks on critical infrastructure in the past, claimed responsibility for the sabotage in a letter titled: “Cutting off power to those in power,” which was published online.

“In the greed for energy, the earth is being depleted, sucked dry, burned, ravaged, burned down, raped, destroyed,” the group, which is listed by Berlin’s intelligence services as a left-wing extremist organization, said in the letter.

“The aim of the action is to cause significant damage to the gas industry and the greed for energy,” its authors wrote. The group has used similar means to communicate in the past, and Berlin police believed the letter to be genuine.

With temperatures below freezing in the German capital, schools and kindergartens in the southern districts affected by the power outage remained closed on Monday morning. Around 30,000 households and approximately 1,700 businesses were still without power on the third day of the power outage. Full restoration of supply is expected to take until Thursday.

The city’s energy senator, Franziska Giffey told POLITICO’s Berlin Playbook Podcast on Monday that Berlin’s critical infrastructure needed better protection.

“There is a great deal of public information about our critical infrastructure that we need to publish and make transparent. In the future, we will have to consider how we can handle this differently and how we can protect ourselves even better against these issues,” she said.

In a separate interview with Berlin’s public broadcaster rbb, Giffey said prosecutors at the national level would need to assist with the investigation.

“The question is, are these just left-wing activist groups acting on behalf of ideology, or is there more to it than that? That absolutely must be investigated,” said the politician from the center-left Social Democratic Party that governs Berlin in a coalition with Wegner’s conservatives.

“This is not just an attack on our infrastructure, but also an attack on our free society.”

Josh Groeneveld and Rixa Fürsen contributed to this report.