An elite hacking group sponsored by Russian intelligence gained access to the emails of some of Microsoft’s senior executives beginning in late November, the company disclosed in a blog post and regulatory filing on Friday.

Microsoft said it had discovered the intrusion a week ago and was still investigating. The hackers appeared to focus on combing through Microsoft’s corporate email accounts to look for information related to the hacking group, which Microsoft’s researchers called Midnight Blizzard.

The hackers looked through emails from Microsoft’s senior leadership team as well as employees in cybersecurity, legal and other groups, and took some emails and attachments, the company said. The company, which had worked with cybersecurity firms and governments to investigate previous attacks by the hacking group, did not name the executives whose emails were targeted.

The Russian Foreign Intelligence Service has run the hacking group since at least 2008, according to the U.S. Cybersecurity and Infrastructure Security Agency. The group is known by a variety of nicknames, including Cozy Bear, the Dukes and A.P.T. 29, and has been behind a number of high-profile hacks, according to previous U.S. government investigations.