How could Democrats know Telizhenko was a Russian agent? Either they made it up or someone in the U.S. Government, such as FBI or CIA, told them. If Telizhenko really was a Russian agent he should have been targeted and investigated by FBI counter intelligence officials. Hmm, who had those duties at the FBI. Oh yeah, some guy named Peter Strzok.
So if you are Peter Strzok and you learn that this guy, Telizhenko, is a Russian agent, what should you do? How about identifying his network without tipping him off? Maybe try entrap him? Or here is a wild thought, try to recruit him, flip him, and put him to work spying for the United States. That is what a professional counterintelligence agent would do. But Strzok apparently did not do any of that. The only thing we know is that the FBI told at least one member of Congress that Telizhenko could not be trusted. Why?