The FBI briefed both the Trump and Clinton campaigns about ongoing Russian attempts at infiltrating and influencing them. They were also told to alert the FBI about any “suspicious overtures.” Trump later claimed that he received no such warning.

A great deal of attention had already been drawn to the connections between the Trump campaign and Russians. More overt examples include Trump’s past business dealings and calling for Russia to hack Clinton’s emails. Several members of his campaign were also convicted of perjury by hiding Russian connections to the FBI, including General Michael Flynn, who Trump named a national security advisor after the election.

NBC News – FBI warned Trump in 2016 Russians would try to infiltrate his campaign
CNN – Trump says he wasn’t warned in 2016 about Russia threats. He was.
Wikipedia – Links between Trump associates and Russian officials