Tweet follows appointment of special prosecutor to head Russia investigation
"This is the single greatest witch hunt of a politician in American history!," Trump wrote on Twitter just hours after the Justice Department announced former FBI Director Robert Mueller would lead the bureau's ongoing probe.
The decision comes amid mounting concerns about Trump's potential attempts to interfere in the investigation, including the sacking of former FBI Director James Comey last week.
Multiple news outlets reported Tuesday that prior to ousting Comey, Trump asked him to terminate an investigation into former National Security Advisor Michael Flynn.
In announcing the appointment of a special counselor, Deputy Attorney General Rod Rosenstein said he determined the move was "in the public interest".
But Rosenstein's decision prompted further outrage from Trump.
"With all of the illegal acts that took place in the Clinton campaign & Obama Administration, there was never a special counsel appointed!," Trump wrote.
Mueller served as FBI director under Democratic and Republican administrations from 2001 to 2013, and is highly respected in Washington.
He served beyond the normal 10-year term for directors in order to allow time for Comey to be confirmed.
He will resign his post at a private law firm in order to avoid potential conflicts of interest, the Justice Department said.