A team of sophisticated hackers believed to be working for the Russian government won access to internal communications at the US Department of Homeland Security, people familiar with the matter told Reuters on Monday.

According to the report, the breach was part of Sunday’s cyberattack which penetrated the US departments of Treasury and Commerce.

During Sunday’s attack, information was stolen from the US Treasury Department and the Commerce Department’s National Telecommunications and Information Administration (NTIA).

“The United States government is aware of these reports and we are taking all necessary steps to identify and remedy any possible issues related to this situation,” National Security Council spokesman John Ullyot said.

The Washington Post, citing people familiar with the matter, reported that Russian government hackers are responsible for the breach.

The same hackers earlier breached a top cybersecurity firm, said the sources who requested anonymity because of the sensitivity of the matter.