(CNN) The Republican leaders of two House committees say their investigation into the FBI's handling of Hillary Clinton's email server and its probe of alleged coordination between the Trump campaign and Russia uncovered concerns about the "thoroughness and impartiality" of each investigation.

House Judiciary Chairman Bob Goodlatte and Oversight Chairman Trey Gowdy sent a six-page letter Friday summarizing the findings of their joint probe into the FBI and Justice Department, writing that further investigation was needed -- including appointing a second special counsel -- to examine the "disparate way these two investigations were seemingly conducted."

Notably, the Republican lawmakers also argued that their investigation was not an attempt to undercut special counsel Robert Mueller -- a charge leveled by Democrats throughout the Republican-led investigation.

"Quite the opposite, whatever product is produced by the special counsel must be trusted by Americans and that requires asking tough but fair questions about investigative techniques both employed and not employed," Goodlatte and Gowdy wrote.

The letter, which was addressed to acting Attorney General Matt Whitaker, DOJ Inspector General Michael Horowitz and Senate Majority Leader Mitch McConnell, marks the conclusion of the yearlong Republican investigation.

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