BBC News reports that a power cut that hit part of the Ukrainian capital, Kiev, in December has been judged a cyber-attack by researchers investigating the incident.
The blackout lasted just over an hour and started just before midnight on 17 December.
The cyber-security company Information Systems Security Partners (ISSP) has linked the incident to a hack and blackout in 2015 that affected 225,000 people.
It also said a series of other recent attacks in Ukraine were connected.
The 2016 power cut had amounted to a loss of about one-fifth of Kiev’s power consumption at that time of night, national energy company Ukrenergo said at the time.
It affected the Pivnichna substation outside the capital, and left people in part of the city and a surrounding area without electricity until shortly after 01:00.
The attack took place almost exactly one year after a much larger hack on a regional electricity distribution company. That was later blamed on the Russian security services.
The latest attack has not publicly been attributed to any state actor, but Ukraine has said Russia directed thousands of cyber attacks towards it in the final months of 2016.
ISSP, a Ukrainian company investigating the incidents on behalf of Ukrenergo, now appears to be suggesting a firmer link.
It said that both the 2015 and 2016 attacks were connected, along with a series of hacks on other state institutions this December, including the national railway system, several government ministries and a national pension fund.
In December, Ukraine’s president, Petro Poroshenko, said hackers had targeted state institutions some 6,500 times in the last two months of 2016.
He said the incidents showed Russia was waging a cyber-war against the country.