Speaking on CNN’s “State of the Union,” Schiff said that lawmakers are in talks with Mueller’s office about testifying, but that time is “running out” for him as the August recess for Congress looms on the horizon.

“We have been in private discussions with the special counsel’s office,” Schiff said. “I’d hope we reach this decision this week.”

Schiff added: “We’ll reach a final conclusion; either he’s going to come in voluntarily or we’re going to have to subpoena him.”

In a rare press conference last month, Robert Mueller said he had fulfilled his duties as special counsel investigating Russian interference during the 2016 presidential election and had no intention of appearing before Congress.

“I am speaking out today because our investigation is complete,” Mueller said last month. “The attorney general has made the report on our investigation largely public. We are formally closing the special counsel’s office. And as well, I’m resigning from the Department of Justice to return to private life.”

“I hope and expect this to be the only time I will speak to you in this matter,” Mueller added. “No one has told me whether I can or should testify or speak further about this matter.  There has been discussion about any appearance before Congress,” he said.

“Any testimony from this office would not go beyond our report. It contains our findings and analysis, and the reasons for the decisions we made. We chose those words carefully, and the work speaks for itself.”

However, House Democrats, have been insistent that Mueller testify, with House Judiciary Chairman Jerry Nadler, D-N.Y., also putting bold threats to subpoena the special counsel if he doesn’t voluntarily appear before Congress. Nadler’s committee members said they are still negotiating with Mueller’s team and that no timeline for an appearance is yet clear.