Mueller reports to Deputy Attorney General Rod Rosenstein, who appointed him, although the rule gives a special counsel broad discretion to run the investigation without regularly consulting with the department or requesting its approval. Mueller also reports in addition to his office, the Justice Department has spent $3.5 million to support the investigation.
The combined $6.7 million spent by Mueller's office and other offices supporting his investigation only covers the special counsel's investigation, not the various congressional investigations of Russia's meddling in the election.
Two other campaign officials, former campaign chairman Paul Manafort and his associate Richard Gates, also face charges in the investigation. The Justice Department's report covered the period from May 17 to September 30.
Equipment acquisitions marks the second-highest expense, coming in at more than $733,000, followed by costs for rent and utilities, travel and transportation, and contractual services.
Of that price tag, only about $3.2 million was spent directly by the special counsel's office. On average, according to FiveThirtyEight, previous special counsel investigations that have filed criminal charges have taken more than a year to do so.
On December 1, Mueller reached a plea agreement with former national security adviser Michael Flynn, who pleaded guilty to giving misleading the Federal Bureau of Investigation about his contacts with then Russian Ambassador to the US, Sergey Kislyak. He spent $26,592 in the probe's first phase, from the time he was appointed on December 30, 2003 to March 31, 2004, on personnel, travel, contracts and supplies, according to an audit from the Government Accountability Office.
A day later, the DOJ announced that former Trump campaign aide George Papadopoulos pleaded guilty to lying to federal agents during the investigation about his contacts with three individuals said to have had connections to the Russian government.
Mueller reported $1.7 million in personnel compensation and benefits for members of his team.