Although the government had an inklings of a plot, the first clear intelligence came with the anonymous warning given to a Catholic nobleman, Lord Monteagle, that he should not attend the opening of Parliament on 5 November. The letter is held by The National Archives.
What the letter said
My Lord…I would advise you as you tender your life to devise some excuse to shift your attendance at this Parliament for God and man hath concurred to punish the wickedness of this time…though there be no appearance of any stir yet I say they shall receive a terrible blow this parliament and yet they shall not see who hurts them.
This is a warning, telling Lord Monteagle to stay away from Parliament as something terrible was going to happen, something which nobody would see coming.
Monteagle received the letter at his house in Hoxton, north London, on 26 October 1605, and immediately passed it to Robert Cecil, Earl of Salisbury, the King's most important minister. It is not known who wrote the letter, but the most likely candidate is Francis Tresham, Monteagle's brother-in-law.
Another conspirator, Thomas Winter, was well known in Monteagle's household, but it has been claimed that the letter was sent by someone in, or linked to, the government.
It may even have been Salisbury himself, in which case the plotters had been set up to provide a spectacular propaganda coup for the government.