Washington (CNN) Sen. Ron Johnson asked the FBI Thursday to answer more questions about its handling of a closed investigation now drawing renewed attention: the Hillary Clinton private email server probe.

The Wisconsin Republican asked FBI Director Christopher Wray for more information about edits to former director James Comey's famous statement on Clinton's use of a private email server the summer before the 2016 election.

In his letter to Wray , Johnson cites an early draft of Comey's statement, which he says shows "repeated edits to reduce Secretary Clinton's culpability in mishandling classified information."

Johnson notes in his letter to Wray that the term "gross negligence" is "a legal threshold."

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