Under interrogation, Dereham admitting pre-marital affairs with Katherine, but claimed that they were never intimate after Katherine's marriage to the King. Furthermore, he claimed that he had been supplanted in her affections by Culpepper — this is interesting, considering the necessary secrecy of Katherine's relationship with Culpepper. Any incriminating documents were most likely burnt by Agnes, dowager duchess of Norfolk, as it is documented that she raided Dereham's coffers and destroyed letters. However, Cranmer was faced with the rumours of a pre-contract of marriage between Dereham and Catherine, which was effectively as binding as marriage itself, especially if the couple sealed the agreement with sexual relations. If this was true (there is no evidence to either prove nor disprove the allegation), Katherine's marriage to Henry would have been unlawful. A supposed love letter from Katherine to Culpepper had been discovered, sealing her fate and all those implicated.

Dereham died a traitor's death at the Tyburn gallows, being hanged, drawn and quartered. Culpepper also died at Tyburn, but as he had been favoured by the King before his affair with Katherine, his sentence was commuted to beheading. Katherine was beheaded at the Tower of London on 13 February 1542. Agnes, dowager duchess of Norfolk was eventually released.

In a confession, in the form of a letter of 7 November 1541 to the King, Katherine wrote the following regarding her relationship with Dereham:

…Francis Derehem by many persuasions procured me to his vicious purpose, and obtained first to lie upon my bed with his doublet and hose, and after within the bed, and finally he lay with me naked, and used me in such sort as a man doth his wife, many and sundry times, and our company ended almost a year before the King's Majesty was married to my Lady Anne of Cleves [Henry's preceding wife] and continued not past one quarter of a year, or a little above…