Former FBI Director James Comey James Brien ComeyDemocrats fear Russia interference could spoil bid to retake Senate Book: FBI sex crimes investigator helped trigger October 2016 public probe of Clinton emails Trump jabs at FBI director over testimony on Russia, antifa MORE questioned Wednesday why special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE did not subpoena President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE during his nearly two-year-long investigation into Russian election interference and possible collusion between the Trump campaign and Moscow.

Asked by NBC's Lester Holt if he ever questioned why Mueller did not subpoena Trump, Comey responded “Yes, I do.”

“And I don’t know the answer to that,” he added.

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Comey's comments came days after Mueller submitted his investigation's findings to the Justice Department.

In a four-page memo presented to Congress on Sunday, Attorney General William Barr Bill BarrProsecutor says no charges in Michigan toilet voting display Judge rules Snowden to give up millions from book, speeches The Hill's Morning Report - Sponsored by Facebook - Washington on edge amid SCOTUS vacancy MORE said Mueller's probe did not find sufficient evidence to conclude the Trump campaign had coordinated with Russia. Mueller's report, he said, did not exonerate Trump.

Trump was not interviewed by Mueller during the sweeping 22-month long probe, despite the special counsel's office seeking a sit-down with the president for more than a year. The president instead submitted written answers to certain questions from Mueller related to the Russia investigation in November.

Comey said Tuesday that he found it "confusing" that Mueller decided not to determine whether Trump was guilty of obstruction of justice.

"The notion that obstruction cases are somehow undermined by the absence of proof of an underlying crime, that is not my experience in 40 years of doing this, nor is it the Department of Justice's tradition. Obstruction crimes matter without regard to what you prove about the underlying crime," Comey said.

Comey led the Russia probe before Trump fired him in May 2017.