How does Mrs. Rogers die?

From the text:

"No, sir. Wargrave and Lombard were shot, the first through the head, the second through the heart. Miss Brent and Marston died of cyanide poisoning, Mrs. Rogers died of an overdose of chloral. Rogers'head was split open. Blore's head was crushed in. Armstrong died of drowning. Macarthur's skull was fractured by a blow on the back of the head and Vera Claythorne was hanged."