Washington (CNN) There were few people who despised James Comey more than those political operatives working for Hillary Clinton in Brooklyn during the 2016 campaign. But word of the FBI director's firing was met with fear, not joy, on Tuesday.

Sen. Tim Kaine, Clinton's running mate, Robby Mook, the former secretary of state's campaign manager and Brian Fallon, her press secretary, all expressed skepticism at why Comey was fired now -- in the midst of the bureau's investigation into Russia's involvement in the 2016 election.

"Trump firing Comey shows how frightened the Admin is over Russia investigation," Kaine tweeted in response to the news. "Comey firing part of a growing pattern by White House to cover-up the truth."

Mook, who has been markedly critical of Comey since the 2016 election, tweeted that he was "surprised" by the fact that he can't see "how this bodes well for the Russia investigation."

"I was as frustrated, concerned and disappointed as anyone with Director Comey's handling of the email investigation, but President Trump just fired the man investigating how Russia meddled in our election and whether members of his campaign were involved, an investigation President Trump called 'charade' only 24 hours ago," Mook said in a statement. "It's equally concerning that our attorney general, who lied about his own meetings with the Russians, approved Director Comey's firing."

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