For two days after the death of Edward VI, the Duke of Northumberland kept the news secret while he put the next stages of his plan into action. Urgent messages were sent to both princesses, Mary and
Elizabeth, bidding them to come to London to see their brother before he died. Always wary, Elizabeth suspected danger and stayed safely where she was, pleading ill-health. However, Mary did as she was told and set out. Only the timely arrival of a secret message, sent by Sir Nicholas Throckmorton, saved her from falling into Northumberland's trap.
 

Source: Leo van de Pas