This is a pretty good article on how congressional investigators, government watchdog groups and others think that the private investigators and researchers that worked on the Mueller report could be Christopher Steele and Fusion GPS.
https://www.realclearinvestigations.../who_were_the_mueller_reports_hired_guns.html
https://www.realclearinvestigations.../who_were_the_mueller_reports_hired_guns.html