Washington (CNN) Emails obtained by CNN show the FBI agent at the center of a Capitol Hill storm played a key role in a controversial FBI decision that upended Hillary Clinton's campaign just days before the 2016 election: the letter to Congress by then-FBI Director James Comey announcing the bureau was investigating newly discovered Clinton emails.

The new revelation about FBI agent Peter Strzok comes as Republicans accuse him of being sympathetic to Clinton while seeking to undermine Donald Trump during the heat of the 2016 campaign season.

Strzok, who co-wrote what appears to be the first draft that formed the basis of the letter Comey sent to Congress, also supported reopening the Clinton investigation once the emails were discovered on disgraced former Rep. Anthony Weiner's laptop, according to a source familiar with Strzok's thinking. The day after Strzok sent his draft to his colleagues, Comey released the letter to Congress, reigniting the email controversy in the final days of the campaign.

Strzok did, however, harbor reservations about Comey making a public announcement just days before the election and sent a text message to that effect, two sources said. And Strzok's text messages provided to Congress show him grappling with the fallout of making the letter public, according to a CNN review of his texts.

This new information reveals a more complicated portrait of Strzok than many of his critics have painted in public. Republicans have seized on text messages between Strzok and FBI lawyer Lisa Page, who were often critical of Trump, to characterize the two -- who were having an extramarital affair -- as part of an effort to go easy on Clinton and get tough with Trump.

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