After I left graduate school, my advisor (the corresponding author) and another person (the first author) published a paper with a lot of data from my master's thesis. I was not even acknowledged, and I feel this may constitute plagiarism. Normally I would reach out to my former advisor, but unfortunately, they have passed away.
So: how should I proceed? Who would deal with issues and questions pertaining to authorship and plagiarism in this circumstance?