Mark Cuban’s brother has posted online a series of fiery e-mails between Mr. Cuban and Jeffrey Norris, a Fort Worth lawyer with the U.S. Securities and Exchange Commission – the same government agency that accused Mr. Cuban of insider trading this week.

In the lengthy correspondence, which began in March 2007, Mr. Norris criticized Mr. Cuban for his potential involvement with a controversial movie called Loose Change.

Mr. Cuban, the billionaire owner of the Dallas Mavericks, reportedly discussed distributing the film but ultimately did not. Loose Change argued that people within the U.S. government planned the 9/11 terror attacks.

“I had become a passionate Maverick fan, following every game and letting the team’s success or failure in the playoffs practically control my entire emotional state,” said Mr. Norris, in an e-mail to Mr. Cuban on March 28, 2007.

“Your association with Loose Change has altered that,” Mr. Norris wrote. “You have chosen to promote opinions that slander our country and our president.”

Mr. Cuban replied, “Explain to me how this movie … will help people who hate this country. And to question my patriotism, well, you have no clue.”

The SEC declined to comment on the e-mails, but it has said that Mr. Norris played no role in its investigation of Mr. Cuban, which began in late 2006 or early 2007. Mr. Norris could not be reached for comment.

In a lawsuit filed this week in Dallas, the SEC accused Mr. Cuban of selling shares in an Internet company in June 2004 after learning confidential information about the firm and agreeing not to act on it.

He saved $750,000 by selling the shares before the sensitive information became public, the SEC alleges.

Mr. Cuban denies wrongdoing and says he never agreed to keep the information he learned confidential. A lawyer for Mr. Cuban declined to comment about the e-mail exchange with Mr. Norris.

The claims of Loose Change have been dismissed by major media outlets.