On the 24th floor of Trump Tower, in an office two floors below Donald Trump, Felix Sater was trying to revive his career. The Russian-born businessman had already done a stint in prison for stabbing a man in the face with the stem of a margarita glass, and he was now awaiting sentencing for his role in a Mafia-orchestrated stock fraud scheme — all the while serving as a government informant on the mob and mysterious matters of national security....

Trump and his lawyers have said that he was not aware of Sater’s criminal past when he first signed on to do business with Sater’s firm, Bayrock Group. Sater’s involvement in the stock fraud was kept secret for years by federal prosecutors because of his role as an informant.

But even after elements of Sater’s background were disclosed in a 2007 New York Times article, he remained in close proximity to Trump — at one point using Trump Organization office space and business cards.