Here is a modern bit of analysis that comes as close to being empirical evidence as anything in this story. And it points directly at the prime suspect.
The following is from the website: Guy Fawkes: Was there a Gunpowder Plot?
"Miss Joan Cambridge, a professional document examiner accustomed to giving expert evidence in courts of law, then chairman of Scientific Graphologists (England) and one of Britainís leading authorities on handwriting, was asked to make a microscopic examination of original documents at the Public Record Office and to report, as if in a forgery case. Officials at the Public Record Office helped by removing the Monteagle letter from their museum for her to examine.
"Here are Miss Cambridgeís answers to a series of questions.
- Was the Monteagle letter written by somebody attempting to disguise his hand? - Yes, beyond doubt.
- In traditional accounts Francis Tresham or the Jesuit Father Garnett have been suggested as possible authors. Could either have written it? - Extremely unlikely for Tresham to have written it and virtually impossible for Garnett to have done so.
- Could Cecil have written it? - Yes, beyond doubt.
- Did Cecil write it? - Probably he did. It is a 70 per cent probability. I canít go further than that because there are special factors of the ink and paper having aged. But Iím reasonably satisfied in my own mind that he did."