The power grid in the eastern German state of Brandenburg was the target of sabotage on Tuesday after unexploded incendiary devices damaged power lines, the state's interior minister said.
The power grid attack adds to a list of incidents causing Germany to consider how to better protect critical infrastructure from what officials have called a growing threat from cyber and hybrid attacks.
The Turnow-Preilack substation is one of the region's most critical high-voltage nodes, connecting Brandenburg's largest power plant, Jaenschwalde, to the 50Hertz transmission grid.
Rockets carrying conductive material were fired into extra-high-voltage lines, causing two short circuits, Interior Minister Jan Redmann said at a press conference.
He added that authorities were searching the site, and the number of devices found, in the double digits, was still rising.
Redmann did not rule out the possibility that foreign powers were behind the attack, nor did he exclude left-wing activists, who have been involved in previous attacks on power structures.
The rockets were specially designed and, unlike typical celebratory fireworks, he said, required significant setup.
There was a brief power cut, but all power lines are back in operation, and the general electricity supply was never affected, grid operator 50Hertz said in a statement.
Adi Roesch, CEO of LEAG, the power plant operator, said the act caused a spontaneous shutdown of one unit at the power plant, which was expected to be restored by Tuesday evening.
Critical infrastructure needs protection
"When it comes to our critical infrastructure, we in Germany are far too vulnerable," said Redmann, adding that the vulnerability also extended to airports.
Germany blamed Russia on Tuesday for an attempted drone attack on a Ukrainian cargo plane at Leipzig/Halle airport last month and ordered a series of measures in response.
Russia has previously dismissed accusations about its possible involvement in the airport incident as a "fabricated provocation."
Germany on high alert for cyber, hybrid attacks
Germany has been on high alert in recent weeks because officials have described a growing threat from cyber and hybrid attacks.
Brandenburg premier Dietmar Woidke condemned the alleged attack on the substation.
"We take attacks on our critical infrastructure very seriously. The alarmingly high frequency of such incidents shows that we must resolutely protect our infrastructure," he said.
Germany is set to blame Russia for an attempted drone attack on a Ukrainian cargo plane at Leipzig/Halle airport last month and could prepare new sanctions against Moscow in coordination with allies, two sources said on Tuesday.
Russia has previously dismissed accusations about its possible involvement in the airport incident as a "fabricated provocation" and as "a new wave of anti-Russian hysteria in Germany."