"I think there were serious questions raised about how he handled this," Mook said on "Squawk Box." "But the idea that the same day that we find out subpoenas are going out relevant to the Russia investigation into President Trump's campaign, he decides to suddenly fire the director of the FBI because he claims he's upset about he handled Hillary's e-mails."
"The argument is absurd," said Mook, who was critical of Comey on how he handled Clinton's email probe.
CNN reported Tuesday that federal prosecutors issued grand jury subpoenas as part of the probe of Russia interference in the U.S. election.
White House spokesman Sean Spicer said Trump fired Comey "based on the clear recommendations of both Deputy Attorney General Rod Rosenstein and Attorney General Jeff Sessions."
Rosenstein focused on Comey's statements days before the November election about the FBI investigation of Hillary Clinton's email practices. During the final days of the campaign, Trump had praised Comey's announcement, which Clinton blames for her loss to Trump.
Many Democrats called for Comey to immediately be summoned to testify before Congress about the status of the Trump-Russia investigation. Senate Minority Leader Charles Schumer said the Justice Department should appoint an independent prosecutor for the Russia inquiry.
—The Associated Press contributed to this report.