Yes, that part is still undisclosed by Microsoft. It is mentioned in the article as well:
We will continue to closely monitor this incident and provide updates; this is still an ongoing investigation and there are many unanswered questions (how did the threat actor acquire the key? When exactly did it happen? Were other keys compromised as well?).
I still get goosebumps from so many of her songs, wonderful artist as well as a troubled soul.
RIP