The whistleblower at the center of Democrats' impeachment inquiry acknowledged to the Intelligence Community Inspector General (ICIG) that bias against President Trump might be alleged against him or her for a third, previously unreported reason, sources familiar with the ICIG investigation tell Fox News.

Fox News has previously reported the whistleblower is a registered Democrat and had a prior work history with a senior Democrat.

Though Fox News has learned that an additional element of possible bias was identified by the whistleblower, its nature remains unclear.

The whistleblower has insisted the complaint was not politically motivated, Fox News is told. Critics note, however, that the whistleblower met with House Intelligence Committee Chairman Adam Schiff’s staff before the complaint was filed around mid-August.

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Yet on a disclosure form, the whistleblower did not check the box indicating that he or she had contact with “Congress or congressional committee(s)" before filing the complaint.

Schiff had previously claimed in a televised interview that "we have not spoken directly with the whistleblower" prior to the complaint being filed.

A Schiff spokesperson narrowed that claim earlier this month, telling Fox News that Schiff himself "does not know the identity of the whistleblower, and has not met with or spoken with the whistleblower or their counsel" for any reason.

An aide to Schiff insisted that when Schiff mentioned "we" had not spoken to the whistleblower, he was referring to members of the full House intelligence committee, rather than staff.

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Fox News is told the disclosure form does acknowledge contact with “Other Office of Department/Agency involved,” which is presumed to be the CIA.