In fact, F.B.I. agents sent an informant to talk to two campaign advisers only after they received evidence that the pair had suspicious contacts linked to Russia during the campaign. The informant, an American academic who teaches in Britain, made contact late that summer with one campaign adviser, George Papadopoulos, according to people familiar with the matter. He also met repeatedly in the ensuing months with the other aide, Carter Page, who was also under F.B.I. scrutiny for his ties to Russia…

Details about the informant’s relationship with the F.B.I. remain scant. It is not clear how long the relationship existed and whether the F.B.I. paid the source or assigned the person to other cases.

Informants take great risks when working for intelligence services, Christopher A. Wray, the F.B.I. director, testified before Congress on Wednesday. Their identities must not be exposed, he said, hinting at congressional efforts to obtain the name of the source. “The day that we can’t protect human sources is the day the American people start becoming less safe.”