Sally Yates, fired Jan. 30, 2017: As acting AG, Yates was key to the investigation into Trump’s aides and their potential connections to Russia — she was collecting intelligence on the Russian ambassador to the U.S. and which members of the Trump team he had been in contact with. (That includes Mike Flynn, who was fired due to his communications with the ambassador.)
Preet Bharara, fired March 11, 2017:(Former U.S. Attorney) Bharara was investigating Trump’s HHS Secretary Tom Price for his financial investments. Bharara was also investigating corrupt Russian businessmen and officials (and a witness for the case was pushed or fell from a window the day before he was set to testify in another court case).
James Comey, fired May 9, 2017: Comey was, like Yates, investigating Trump’s Russia ties, which he revealed on March 20.
“Days before he was fired, Comey asked for added resources for the FBI’s Russia investigation. According to two Senate aides with knowledge of the investigation, last week Comey met with Deputy Attorney General Rod Rosenstein to ask for significant added resources for the Russia investigation. On Monday, Comey briefed key senators about the request.”