The US Cybersecurity and Infrastructure Security Agency (CISA) said Russian government-backed hackers used their access to Microsoft’s email system to steal correspondence between officials and the tech giant, according to an emergency directive from US regulator published on Thursday (April 11).

In the directive, dated April 2, the agency warned that hackers were exploiting email credentials to try to break into Microsoft customer systems, including those of an unspecified number of government agencies.

The warning that government agencies are being targeted using stolen Microsoft emails follows the company’s announcement in March that it is still battling the attackers, which it calls the “Midnight Blizzard.”

That revelation, which has sent alarm bells ringing across the cybersecurity industry, was followed just last week by a report from the US Cybersecurity Review Board, which said a separate hack – attributed to China – was able to avoided, accusing the company of cybersecurity lapses and a deliberate lack of transparency.

CISA declined to name the organizations that may have been affected. Microsoft said in an email that it is “working with our customers to help them investigate and mitigate the impact. This includes working with CISA on an emergency directive to provide guidance to government agencies.”

The Russian embassy in Washington, which has previously denied being behind hacking campaigns, did not immediately respond to a message seeking comment.

CISA warned that the hackers may have also targeted non-governmental groups.

“Other organizations may also have been affected by Microsoft’s corporate email quarantine,” CISA said, encouraging customers to contact Microsoft for more details.