Acting Consumer Financial Protection Bureau director Mick Mulvaney has told Sen. Elizabeth Warren, D-Mass., that he doesn't plan on responding to her questions about the agency, and said it’s her fault that he is not required to answer.

Mulvaney, a conservative who was an outspoken critic of the bureau during his time as a congressman, told Warren in a letter sent Wednesday that the structure of the agency, which she helped design, shields him from accountability.

“I encourage you to consider the possibility that the frustration you are experiencing now, and that which I had a few years back, are both inevitable consequences of the fact that the Dodd-Frank... Act insulates the Bureau from virtually any accountability to the American people through their elected representatives,” Mulvaney wrote.

He told Warren that he wouldn’t be answering any of the 105 unanswered questions she submitted to him about his management of the CFPB and handling of cases against payday lenders and others. Instead, he said he would discuss them during congressional testimony, when Warren will likely have five or 10 minutes to question him in the Banking Committee.

Mulvaney apologetically claimed in his latest letter that one set of questions from Warren more or less got lost in the mail.

Mulvaney and Warren have traded several rounds of hostile or taunting correspondence over his attempts to steer the CFPB in a conservative direction. The situation is reversed from when President Obama’s appointee, Richard Cordray, ran the bureau and implemented several major new rules while suing a range of companies for consumer financial practices.

[Opinion: Elizabeth Warren v. Mick Mulvaney is a five-star, thumbs-up, must-watch political miniseries]

This week, Mulvaney also called on Congress to reform the agency and curb many of its powers.

Conservatives have long argued that the bureau is unconstitutional because, as set up by the Dodd-Frank law, it is run by a sole director who can’t be removed by the president except for cause, and it gets its funding from the Federal Reserve rather than from Congress.