Who was the letter sent to that led to the search of the cellars that revealed the gunpowder plot?

The letter that led to the search of the cellars related to the Gunpowder Plot was sent to Lord Monteagle, a member of the English Parliament. The letter warned him to stay away from the opening of Parliament, raising suspicions about a plot against King James I. This prompted authorities to investigate the cellars beneath the Palace of Westminster, where they discovered the barrels of gunpowder. The plot was ultimately foiled as a result.