Fusion GPS is a Democrat-linked opposition research firm which produced the infamous anti-Trump "Steele Dossier," compiled from a series of memos provided by former MI6 spy Christopher Steele and paid for in part by the Clinton campaign.

Simpson told Congressional investigators on August 22 that Steele told him the FBI had corroborated parts of his dossier with "a human source from inside the Trump organization." 

As the Daily Caller's Chuck Ross notes, Fusion's allies quickly began to backpedal from Simpson's statement, telling news outlets that there was no mole...

"Instead, he was referring to George Papadopoulos, a Trump campaign adviser whose encounter with an Australian diplomat in May 2016 was reportedly the catalyst for the FBI's counterintelligence investigation. The diplomat, Alexander Downer, reportedly claimed that Papadopoulos discussed Russian dirt on former Secretary of State Hillary Clinton." -Daily Caller