Ukraine’s cyber defense agency said on Wednesday that a hacker organization thought to be connected to Russian military intelligence had claimed responsibility for the cyberattack that took down the country’s largest mobile network provider.

The attack on Kyivstar on Tuesday disrupted services, damaged IT infrastructure, and silenced air raid alert systems in some areas of Ukraine. Kyivstar has 24.3 million mobile customers and more than 1.1 million home internet users.

A group of activist hackers, or “hacktivists”, called Solntsepyok said in a post on the Telegram messaging app that it carried out the cyberattack, and published screenshots appearing to show that the hackers had accessed Kyivstar’s servers.

Ukraine’s State Service of Special Communications and Information Protectorate (SSSCIP) said in a statement that an expert group was looking into the incident with the SBU intelligence agency.

“Responsibility for the cyberattack was taken by one of the Russian groups, whose activities are associated with the main directorate of the General Staff of the Armed Forces of the Russian Federation,” it said, referring to Russia’s GRU military intelligence agency.