Musk was told by Demis Hassabis in 2012, see https://time.com/6310076/elon-musk-ai-walter-isaacson-biogra...
Altman read Nick Bostrom's book Superintelligence, he mentions it in this post https://blog.samaltman.com/machine-intelligence-part-1
I also read that book in 2014, and it is how I came to understand this problem early.