Investing.com -- Citron Research founder Andrew Left is set to stand trial in Los Angeles on Monday, facing charges from the US Justice Department that he used social media posts to illegally manipulate stock prices and profit from short positions.

The case centers on Left's posts on X, formerly known as Twitter, about dozens of companies. Prosecutors claim his private communications show he told the public one thing about his trading intentions while doing the opposite.

According to the government, Left opened short positions in Roku Inc. early on January 8, 2019, then posted on Citron's Twitter account at 9:41 a.m. that Roku was "uninvestible," which drove down the stock. Prosecutors say he then falsely claimed to be "watching ROKU from the side," suggesting he wasn't invested, while he made $700,000 from his short position that day.

The government alleges Left didn't actually believe many of his criticisms of companies including Tesla Inc. and American Airlines Group Inc., but intended the posts to be "catalyst" events that moved share prices. He allegedly arranged trades on the anticipated movement ahead of his posts, making $16 million from market manipulation.

Left, 55, has denied misleading investors and said his trades didn't need to be disclosed. His lawyers are expected to argue that his opinions in social media posts fall under free-speech protections.