Robert Mueller takes over FBI’s Russia investigation at pivotal moment

— -- In a surprise move, former FBI Director Robert Mueller was appointed special counsel to oversee the FBI’s investigation into Russian interference in the 2016 presidential election, including potential collusion between Russian agents and members of the Trump campaign, by Deputy Attorney General Rod Rosenstein on Wednesday.

According to the Department of Justice, Mueller will have the authority of a U.S. attorney, including the power to issue subpoenas and bring the matter before a federal grand jury.

"My decision is not a finding that crimes have been committed or that any prosecution is warranted. I have made no such determination,” Rosenstein said in a statement. “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.”

Lawmakers from both sides of the aisle praised the announcement of an independent investigation, while President Donald Trump reacted angrily on Twitter, calling it “the single greatest witch hunt of a politician in American history.”

Mueller takes the reins at a particularly sensitive time for the investigation, days after President Trump fired James Comey as FBI Director and amid reports that Comey says Trump pressured him to end his investigation of former national security adviser Michael Flynn.

This is another dramatic chapter in an investigation that began last summer, after experts traced a series of hacks into the Democratic National Committee and the private email account of Hillary Clinton’s campaign chairman to Russia.

Comey confirmed the existence of the investigation during an appearance before Congress in March. He told members of the House Intelligence Committee he had been authorized by the Department of Justice to reveal that the FBI, “as part of our counterintelligence mission, is investigating the Russian government’s efforts to interfere in the 2016 presidential election, and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts.”

Counterintelligence investigations are undertaken largely in secret and can utilize a range of government tools, including secret recordings, intelligence gathered by spies overseas, and information shared from friendly foreign governments. Based on the limited information available, this investigation appears to be no different.

Late in 2016, however, Sen. John McCain is reported to have given then-FBI Director James Comey a copy of a series of memos about Russian meddling in the campaign prepared by a former British counterintelligence official who was working as a private consultant. The memos, later made public by Buzzfeed, included raw intelligence on possible contacts between Trump campaign officials and Russian agents, and suggested that the Russian government had made compromising videos of Donald Trump during a visit the real estate tycoon made to Moscow in 2013. Some of the most sensational claims in the dossier remain unverified.

The probe into Russian meddling was being run jointly by agents at FBI headquarters in Washington and those based in the Eastern District of Virginia who are under the watch of U.S. Attorney Dana J. Boente, who is also the Acting Assistant Attorney General of the Department of Justice’s National Security Division. The total number of agents involved in the case is not known, but they have reportedly been assisted by intelligence experts from several agencies, including the Central Intelligence Agency.

The agents were reportedly looking at the movements and activities of several Trump associates and campaign aides, including Trump’s former campaign chairman, Paul Manafort, his first National Security Adviser, Gen. Michael Flynn, his longtime friend Roger Stone, and a low-level foreign policy adviser, Carter Page. As ABC News reported earlier this year, Page’s name had surfaced on recordings in a previous FBI spy case in New York, and he was alleged at the time to have been targeted by Russian agents who were recruiting intelligence sources in 2013. He was never accused of wrongdoing in that case, but one of those who sought to recruit him went to federal prison on espionage charges and has since been deported.

The investigation had already been designated a “major case” by Comey, meaning that it had its own funding stream inside the FBI. The team of investigators had all moved in to one secured area – or “war room” – inside the bureau’s headquarters where no one without special clearance is allowed to enter. Mueller, however, will have the authority to reassemble the team as he sees fit, and he has 60 days to submit a budget to Rosenstein for approval. He will get to decide whether to keep the probe housed at the FBI or to move it to an off-site location that could offer even greater separation from the rest of the FBI and Justice Department.

The FBI investigation into Russian meddling is separate from the Congressional investigations that are exploring many of the same questions. The House Intelligence Committee has been hampered by political disagreements and has made slow progress. The Senate Intelligence Committee has run a more robust investigation, including issuing subpoenas for records from Gen. Flynn, and seeking some Trump business records from the U.S. Department of Treasury. Members of the senate committee have made repeated trips to the Central Intelligence Agency to examine top secret documents in a secure setting.

ABC News’ Josh Margolin contributed to this report.