The decision heeds the call of Democrats in Congress to appoint a special prosecutor for the probe, which only grew louder after President Trump fired former FBI Director James Comey.

During his own confirmation hearing, Mr. Rosenstein faced questions about whether he would appoint special counsel to handle the probe. He inherited oversight of the investigation after Attorney General Jeff Sessions recused himself.

Mr. Rosenstein said he determined “it is in the public interest for me to exercise my authority and appoint a Special Counsel to assume responsibility for this matter.

“My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination,” he said in a statement issued Wednesday. “What I have determined is that based upon the unique circumstances, the public interest requires me to place this investigation under the authority of a person who exercises a degree of independence from the normal chain of command.”