Reuters/EPA Copyright: Reuters/EPA Lev Parnas (left) and Igor Fruman Image caption: Lev Parnas (left) and Igor Fruman

Their names have come up several times in this hearing - and Yovanovitch just noted she'd never met them and so wasn't sure why they were trying to get rid of her.

But who exactly are they?

Both men are US citizens. Parnas, 46, was born in Ukraine, Fruman, 53, in Belarus.

They are associates of Rudy Giuliani, Trump's personal lawyer, and they reportedly helped Giuliani connect with Ukrainian prosecutors as he attempted to press Ukraine to investigate unsubstantiated corruption allegations against former Vice-President Biden and his son, Hunter.

They were arrested in October on campaign finance charges unrelated to the Ukraine inquiry.

Fruman and Parnas also allegedly raised thousands of dollars in funds for an unnamed congressman to seek their assistance in removing Yovanovitch. This was allegedly done in part by "the request of one or more Ukrainian government officials", according to the criminal complaint against them.

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