Such a book would naturally attract Browning, who, like his father, had an interest in celebrated criminal cases. In his Memories (p. 338), Kegan Paul records his surprise at a dinner-party where the conversation turned on murder, to find Browning acquainted "to the minutest detail" with every cause célèbre of that kind within living memory.
An Artist's Reminiscences, by R. Lehmann (1894), p. 224.
Rossetti Papers, p. 302.
So the story was told by Dante Rossetti, as recorded by Mrs Gilchrist; she says that she believed the story was told of himself by Carlyle.
The passage specially referred to is in Caponsacchi's monologue, II. 936-973, beginning with "Thought? nay, sirs, what shall follow was not thought."