Sweden’s power grid operator is investigating a data breach after a ransomware group threatened to leak hundreds of gigabytes of purportedly stolen internal data.

State-owned Svenska kraftnät, which operates the country’s electricity transmission system, said the incident affected a “limited external file transfer solution” and did not disrupt Sweden’s power supply.

“We take this breach very seriously and have taken immediate action,” said Chief Information Security Officer Cem Göcgören in a statement. “We understand that this may cause concern, but the electricity supply has not been affected.”

The ransomware gang Everest claimed responsibility for the attack on its leak site over the weekend, alleging it had exfiltrated about 280 gigabytes of data and saying it would publish it unless the agency complied with its demands. 

The same group has previously claimed attacks on Dublin Airport, Air Arabia, and U.S. aerospace supplier Collins Aerospace — incidents that disrupted flight operations across several European cities in September. The group’s claims could not be independently verified.

Svenska kraftnät said it is working closely with the police and national cybersecurity authorities to determine the extent of the breach and what data may have been exposed. The utility has not attributed the attack to any specific threat actor. 

“Our current assessment is that mission-critical systems have not been affected,” Göcgören said. “At this time, we are not commenting on perpetrators or motives until we have confirmed information.”