WASHINGTON (AP) — Former President Bill Clinton and former Secretary of State Hillary Clinton say they will refuse to comply with a congressional subpoena for them to testify in an investigation into Jeffrey Epstein.

The Clintons are slamming a Republican-controlled committee’s attempts as “legally invalid” as GOP lawmakers prepare contempt of Congress proceedings against them. In a letter released on social media Tuesday, the Clinton’s tell the chair of the House Oversight Committee, Republican Rep. James Comer, he’s on the cusp of a process “literally designed to result in our imprisonment.”

Comer says he’ll begin contempt of Congress proceedings next week. It potentially starts a complicated and politically messy process that Congress has rarely reached for.