Elon Musk amended his lawsuit against OpenAI and Sam Altman.Marc Piasecki; Justin Sullivan/ Getty ImagesElon Musk amended his lawsuit against OpenAI and Sam Altman, adding Microsoft as a defendant.Musk included emails between himself and Altman from 2015 in which they discussed starting OpenAI.Musk said OpenAI and Microsoft have formed a "de facto merger."Elon Musk unveiled several emails between himself and fellow billionaire Sam Altman this week as evidence in the Tesla CEO's lawsuit against Altman and OpenAI, shining a light on the casual beginnings of the now-$157 billion company.On Thursday, Musk amended his suit against the AI startup and its CEO, adding Microsoft as an additional defendant and accusing the two companies of forming a "de facto merger" and engaging in anticompetitive practices in the AI sector.A spokesperson for OpenAI did not immediately respond to a request for comment from Business Insider.Musk helped found OpenAI in 2015 but has frequently criticized the company and its leaders in the years since.