Former Turing Pharmaceuticals CEO Martin Shkreli—the reviled “pharma bro” who is facing multiple securities-fraud charges—has invoked the Fifth Amendment in refusing to produce documents subpoenaed by a Senate committee investigation on drug-pricing practices. Sen. Susan Collins (R-ME) tweeted Wednesday that by invoking his Fifth Amendment rights, Shkreli could hinder the Senate Special Committee on Aging investigation. Shkreli later tweeted his response to the senator, “I have valid justification. Are you serious? I have constitutional rights. No wonder trust in the US Government is at a low.” The news of his refusal to cooperate came after it was reported that Shkreli was subpoenaed to testify before the House Committee on Oversight and Government Reform on Jan. 26.