John Gately III’s dispute with his sister over financial matters led to the deadly shooting of his brother-in-law, Mayer Brown partner Stephen Shapiro, a prosecutor said at a Thursday bond hearing in a northern Chicago suburb.

Gately, 66, was charged Wednesday with first-degree murder and attempted murder. A state prosecutor said Thursday that the attempted murder charge was related to Gately’s sister, Joan Gately Shapiro, who is also the wife of Stephen Shapiro, a nationally lauded appellate court advocate who launched Mayer Brown’s U.S. Supreme Court practice and argued 30 times before the nation’s highest court.