Your autopsy revealed that at the time of the attack, Mr. White was indeed already dead.  At first it appeared to be heart failure, but further investigations revealed a chemically induced paralysis.   He was killed with a rare poison made from fermented cactus juice.  The poison initially leaves no trace, but within 48 hours after death it coats the heart muscle with green jelly.

Your discovery introduces multiple possibilities.  Was the murder a team effort?  Was the bludgeoning a coverup?  Since the cactus is native to the American Southwest, Burt Sienna is a prime suspect, but he has an alibi and no known motive.  Mrs. White has motive, and no alibi, but there is no obvious connection between her and the American.