Of course he can’t stop lying.
If he told the truth, they’d have to put him and everyone he worked for, and with, in prison.
Put Clapper, Brennan and all the other signers of the letter under oath before the House Intelligence and Homeland Security/Foreign Affairs Committees and grill them about what they signed and who put them up to it, and then subpoena that traitor(s).
Do not take “No” as an answer, only as an evasion in order Not to tell the whole truth.
If they lie or take the Fifth, cut the budget of each of their office/agency by $100 Million dollars. Let’s see if that gets them talking, esp. if they are threatened by their present or former employer with a loss of all pensions, perks, and security clearances.
Do not let them off the hook for any reason until they fully comply with the committees questions.