A hacking campaign is targeting the energy sector in Europe and the US to potentially sabotage national power grids, a cybersecurity firm has warned.

The group, dubbed “Dragonfly” by researchers at Symantec, has been in operation since at least 2011 but went dark in 2014 after it was first exposed, secretly placing backdoors in the industrial control systems of power plants across the US and Europe.

Now, Symantec reports, the group has resumed operations, apparently working since late 2015 to investigate and penetrate energy facilities in at least three countries: the US, Turkey and Switzerland.

“The Dragonfly group appears to be interested in both learning how energy facilities operate and also gaining access to operational systems themselves, to the extent that the group now potentially has the ability to sabotage or gain control of these systems should it decide to do so,” the cybersecurity firm warns.

Dragonfly’s methods are varied, but all its attacks seem to be focused on researching the inner workings of energy firms. It has been seen sending malicious emails with attachments that leak internal network credentials, which are then used to install backdoors on the network allowing the hackers to take control of computers and systems. They’ve also been seen seeding fake flash updates to install the backdoors and carrying out “watering hole” attacks, hacking third-party websites that were likely to be visited by people working in the energy sector.

Currently, the group appears to be solely in information-gathering mode, but Symantec warns that a quiet beginning is often a prelude to deliberate attempts at sabotage. The latest campaigns “show how the attackers may be entering into a new phase,” Symantec says, “with recent campaigns potentially providing them with access to operational systems, access that could be used for more disruptive purposes in future.”

The researchers are unable to determine who is behind the Dragonfly campaign: some of the code is in Russian, but some is in French, “which indicates that one of these languages may be a false flag.

“Conflicting evidence and what appear to be attempts at misattribution make it difficult to definitively state where this attack group is based or who is behind it,” the report concludes.

Attacks on the energy sector have been increasing in frequency and damage in recent years, with Ukraine in particular being at the receiving end of multiple successful strikes. A blackout in west Ukraine in 2015 was caused by a group called Sandworm, while a second attack took out power in the nation’s capital, Kiev, in late 2016.

But other countries, including Britain and the US, have been subject to quieter attempts at infiltration, according to GCHQ. The agency’s National Cybersecurity Centre warned in July that it had spotted connections “from multiple UK IP addresses to infrastructure associated with advanced state-sponsored hostile threat actors, who are known to target the energy and manufacturing sectors”.