https://www.cnet.com/news/democrats-call-on-robert-mueller-to-testify-about-report/

Nadler sent a letter to Mueller, requesting he testify before his committee by May 23. Separately, Rep. Adam Schiff, chairman of the House Intelligence Committee, sent Mueller a letter asking that he appear before Schiff's committee sometime next month.

Without sending a formal letter, Sen. Elizabeth Warren, a 2020 Democratic presidential candidate, said on Twitter that Mueller should testify before Congress.