Ukraine’s power grid has been knocked offline twice before, in 2015 and 2016, causing widespread blackouts. Russia has long used online attacks alongside traditional warfare; just days before the Russian invasion began on Feb. 24, Ukraine said a cyberattack hit its Defense Ministry, its army and two of its banks.

But experts said the latest hacking — while unsuccessful — was among the most sophisticated cyberattacks they have seen in the war so far. It used a complex chain of malware, including some custom-built to control utility systems, suggesting that Russia had planned the attack over several weeks and intended to maximize the damage by sabotaging computer systems that would be needed to restore the electrical grid.

The attack was scheduled to begin on the evening of April 8 as civilians returned home from work, Ukrainian officials said, and could have made it impossible for them to go about their daily lives or gain access to information about the war. The breach targeted several electrical substations in the country, and had it been successful, it would have deprived roughly two million people of electricity and made it difficult to restore power.