Phillips will assume the role Wednesday leading the largest US attorney's office in the country, which has been historically responsible for some of the most significant and politically sensitive cases the Justice Department brings in the US. 

In recent weeks, prosecutors in the office have brought nearly 300 federal cases following the insurrection at the US Capitol on January 6. Hundreds of other people are still being sought by investigators. 

The office was involved in some of the most tumultuous and controversial decisions made by the Justice Department under President Donald Trump, including a decision by then-Attorney General William Barr to reverse the sentencing recommendation by career prosecutors in the case against Trump ally Roger Stone. 

The outgoing acting US attorney, Michael Sherwin, will remain in Washington for a "brief period" to help ensure a smooth transition overseeing the riot investigation and the prosecutions, the Justice Department official said. Sherwin, who for years worked as a career federal prosecutor on drug trafficking, white-collar and top national security cases, will later return to the US attorney's office in southern Florida, the official said. 

The official could not publicly discuss the personnel matter and spoke to the AP on condition of anonymity. 

Phillips served as US attorney in Washington beginning in October 2015 and was a longtime Justice Department official, having been a senior counsellor to the attorney general and deputy associate attorney general.