President Trump's firing of FBI Director James Comey renewed recommendations that the ongoing investigation of possible collusion between Trump campaign associates and Russian operatives will require the appointment of an independent special counsel.
A special counsel can be appointed via two methods:
1. The Attorney General can appoint a special counsel when the DOJ might face a conflict of interest or if "extraordinary circumstances" occur. However, since Jeff Sessions has recused himself from the Russia investigation, that responsibility would now fall to Deputy Attorney General Rod Rosenstein.
2. Congress could pass a law similar to 1978's Ethics in Government Act — passed in response to Watergate — which mandated that allegations against certain high-level government officials require a special counsel independently selected by a panel of judges from U.S. Circuit courts. (After Bill Clinton's impeachment, the Ethics in Government Act's special counsel provision expired in 1999 and was not reauthorized.) This method would require broad bipartisan support to pass both houses of Congress — enough to override the possibility of a Trump veto when the bill crosses his desk.
The only realistic option for a special counsel appointment will come via Rosenstein, the man who drafted the memorandum that explained why Comey was being fired. Still, it's worth noting that Rosenstein isn't a Trump surrogate — he's been at the Department of Justice for 27 years under five different presidential administrations.