Democratic lawmakers have repeatedly accused Sessions of misleading them in congressional testimony and called on federal authorities to investigate, but McCabe's previously-unreported decision to actually put the attorney general in the crosshairs of an FBI probe was an exceptional move.

One source told ABC News that Sessions was not aware of the investigation when he decided to fire McCabe last Friday less than 48 hours before McCabe, a former FBI deputy director, was due to retire from government and obtain a full pension, but an attorney representing Sessions declined to confirm that.

Last year, several top Republican and Democratic lawmakers were informed of the probe during a closed-door briefing with Deputy Attorney General Rod Rosenstein and McCabe, ABC News was told.