Igor Danchenko, a Russian-born analyst living in the US, was arrested Thursday

Faces five counts of lying to FBI agents about his work on the Steele dossier 

He was the primary researcher for British ex-spy Christopher Steele's dossier

Dossier alleged conspiracy between Trump and Russia, but was never proved

It still became a key piece of evidence used in FBI's Crossfire Hurricane probe

Special Counsel John Durham is investigating the origins of Crossfire Hurricane 

Danchenko has previously denied working as an agent for Russian government 

A key source who provided information to British ex-spy Christopher Steele for his 'dirty dossier' of allegations against Donald Trump has been arrested in the US.