The federal Securities and Exchange Commission is suing Charles Wyly Jr. and Samuel Wyly, Dallas billionaire brothers and top Republican campaign donors, accusing them of making $550 million on insider trading.

According to the suit filed Thursday in federal court in New York, the Wylys set up a complicated and fraudulent network of companies and trusts in the Cayman Islands and the Isle of Man to sell stock valued at more than $750 million in companies on whose boards they sat. The brothers failed to disclose the profits made from the scheme, according to the suit.

Data from the Center for Responsive Politics and its website opensecrets.org show the Wylys and their wives have been among the most generous donors to conservative candidates and issues in the country over the last 20 years. The two couples donated almost $2.5 million, more than $1.3 million of it to the Republican National Committee, during that time.