Russian hackers briefly hijacked a dam in Norway in early April and spilled millions of gallons of water before the attack was stopped, Norway’s spy chief revealed Thursday.

The hackers opened a floodgate at the Bremanger dam in western Norway to release the equivalent of about three Olympic-sized swimming pools of water during the four hours they had control of the dam’s computer systems. 

Beate Gangås, the head of Norway’s security police service, blamed the cyberattack on Russian hackers during a speech on Thursday, reports Norwegian media.

The Russian embassy reportedly denied involvement in the breach, per Reuters.

This is the latest incident allegedly involving Russian hackers sabotaging Western energy systems in recent years. Russia was previously blamed for cyberattacks on Ukraine’s power grid that led to widespread blackouts in 2015 and 2016. Russia-backed hackers have long targeted the energy sector.