Mark Cuban, the Internet entrepreneur turned owner of the Dallas Mavericks basketball team, has never shied from a fight. But now the pugnacious billionaire is squaring off against his biggest adversary yet: the federal government. On Monday, the Securities and Exchange Commission filed a civil suit charging Mr. Cuban with insider trading for selling shares of a small Internet search company in 2004, just before its share price fell. [Allegedly] Mr. Cuban saved himself a $750,000 loss. Scott W. Friestad, the S.E.C.'s deputy director of enforcement, said the investigation of Mr. Cuban's trading began in early 2007, but declined to say what had set off the inquiry. A person close to Mr. Cuban provided what he said was one of a series of e-mail messages from Jeffrey B. Norris, an S.E.C. lawyer in Fort Worth, who accused the billionaire of being unpatriotic for helping to finance a movie named Loose Change." In the e-mail message, Mr. Norris described the movie as a "vicious and absurd documentary" that "posits that President Bush planned the demolition of the World Trade Center as a pretext for going to war against Iraq." In the e-mail message, sent from his S.E.C. e-mail address, Mr. Norris said he was informing Christopher Cox, the chairman of the S.E.C., of Mr. Cuban's actions. "If this upsets you, I wonder how George Bush feels," Mr. Norris wrote. "I assume that Mr. Cox would view your involvement with 'Loose Change' much as I do. After all, he served his country as a Republican congressman from Orange County for nearly 20 years and was appointed by President Bush."