Samuel and Charles Wyly, the billionaire brothers from Dallas who are large donors to philanthropies and to conservative causes, were charged Thursday with conducting an extensive securities fraud that the Securities and Exchange Commission said reaped $550 million in undisclosed gains.

The brothers, who founded Sterling Software, a business software and services company that they sold for $4 billion in stock to the software company CA in 2000, were also charged with insider trading violations from which they profited by more than $31 million, the S.E.C. said

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