Russia is believed to have hacked the Europian Union’s embassy in Moscow in a sophisticated cyberespionage attack designed to steal highly sensitive material from the mission’s internal network just weeks before the European Parliament elections.

The initial attack took place in February 2017, but wasn’t detected until April of this year.

European officials aren’t yet sure exactly how much information was stolen at the time, but have determined the attack was continuous, clandestine, and sophisticated hacking technique used to gain access to a system and remain undetected for an extended period of time, according to New Europe.

“We have identified signs potentially pointing to compromised systems connected to our unclassified network in our Moscow Delegation,” an EU spokesperson told the publication. “Measures have been taken to ensure the security of our network and an investigation is in progress.“