The blackout, which hit a large part of the western region of Ivano-Frankivsk on December 23, was due to a computer virus, they said.
The local electricity company, Prikarpattiaoblenergo, said at the time that the breakdown was caused by "the intervention of unauthorised persons ... In the remote access system" and its technicians had had to restore power manually.
But Ukraine's SBU security service later said it found malware -- programmes designed to take over or damage systems -- on the networks of several regional electricity companies.
A spokeswoman for the Ivano-Frankivsk SBU office Maria Rymar, said the agency was still working on the case.
"For the moment, we can't say who did it and for what purpose," she said.
The IT security firm ESET pinned the blame on a programme called KillDisk that was introduced onto the electricity company's computers on an infected Excel spreading document via "phishing" -- tempting an employee to open an inocuous-looking file.
"It was a world first" in bringing down civilian infrastructure, ESET's French subsidiary said in a statement.
"This attack can only confirm what professionals have been fearing -- cyber-criminals are more and more powerful and cyber-attacks will be more and more numerous in 2016."
IT experts have been warning for years about cyber-security in vital civilian infrastructure such as power grids and transport.
Iran's nuclear refining facilities were hobbled in 2010 by a virus called Stuxnet, which is suspected to have been developed by the United States and Israel.
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