However, Bromley played a notable part in 1571 trial of Thomas Howard, 4th Duke of Norfolk, suspected of collusion with the Northern Earls and of involvement in the Ridolfi plot. Bromley spoke after Nicholas Barham, the Queen's Serjeant, who led the prosecution, and Gilbert Gerard, the Attorney General. He was apparently very zealous to secure a conviction. After evidence had been given about the proposed marriage between Norfolk and Mary, Queen of Scots, Bromley's focus was on the communications between Ridolfi and Norfolk. His only evidence of Norfolk's involvement in a plot to invade England and remove the queen was an alleged deciphered copy of a letter given to one Barker to deliver to the Duke. As Norfolk himself argued, there was no evidence that he had ever received the letter, much less that he approved its contents. Bromley was forced to resort to hearsay evidence: that a foreign ambassador in Flanders had heard about the plot and one of his servants had mentioned it to an unnamed English government minister. None of the links in this chain of intelligence gathering was available to give evidence or face questioning but Bromley's allegations were accepted as having explicit royal warrant.