Investigating campaigns

Because Trump and members of his campaign had business dealings with Russia starting many years ago, the FBI began an investigation to determine if these relationships later referred to as collusion affected the election. They hired a professor named Stefan Halper to contact three people in the Trump campaign and gather information from them. He’s been identified by The Washington Post as the ‘informant’ who contacted three Trump campaign officials in 2016 before “he began providing information to the FBI.” Since the FBI did not inform Trump of this, it is alleged by some that they were in fact considering Trump as a suspect in this collusion not just someone on his staff and therein lies the concern. Can an outgoing president use the power of the FBI to investigate the campaign of the incoming president?