The special counsel investigation into possible coordination between President Donald Trump's campaign and Russia during the 2016 presidential election has cost more than $6.7 million so far, according to a financial report released Tuesday.

The release of the report by special counsel Robert Mueller's office comes as the investigation appears to be gaining steam.

Prosecutors have gained a key cooperator in their investigation and revealed that they are keenly focused on the actions of the president and his inner circle.

Of the overall price tag, only about $3.2 million was spent directly by the special counsel's office. An additional $3.5 million was paid out by the Justice Department to support the investigation, though the special counsel's office says that money would have been spent on ongoing probes anyway, even if Mueller had not been appointed.

Pictured: Robert Mueller in 2013. Mueller's Russia investigation has cost more than $6.7 million so far

Prosecutors have gained a key cooperator in their investigation and revealed that they are keenly focused on the actions of President Trump, pictured, and his inner circle

Mueller incorporated several active investigations within the Justice Department including those of Trump campaign contacts with Russia, former Trump campaign chairman Paul Manafort's business activities and former national security adviser Michael Flynn.

The details of the expenditures related to Mueller's investigation were laid out in a report released publicly by the special counsel's office. The report covers from May 17, the date of Mueller's appointment, through Sept. 30, the end of the federal fiscal year.

According to Mueller's report, the special counsel's office spent about $1.7 million for salaries and benefits and more than $223,000 for travel-related expenses. The majority of the travel costs stemmed from the relocation of Justice Department employees temporarily assigned to the expanding investigation.

The office also spent nearly $734,000 on equipment and about $363,000 on rent, communications and utilities.

Previous special counsel investigations, including probes of President Bill Clinton in the late 1990s, have also spent millions over a few months' time.

A 1999 General Accounting Office report, for example, showed that independent counsel Kenneth Starr's office spent $6.2 million in the last six months of 1998, though it's unclear if that amount included both direct expenditures by the special counsel and supporting agency costs. In the report released Tuesday, Mueller's office noted that previous special counsels only reported direct costs and not those incurred separately by the Justice Department on their own expenditure reports.

Paul Manafort accompanied by his lawyers, arrives at U.S. Federal Court, in Washington last month

The four-and-a-half year investigations headed by Starr and his successor, Robert Ray, cost more than $52 million in taxpayer funds as they probed Clinton and then-first lady Hillary Clinton.

A 2000 General Accounting Office report that detailed those figures said that $36 million came from congressional appropriations and $16 million from other federal agency costs. Starr's predecessor, Robert Fiske, spent an additional $6 million investigating the Clintons' involvement in the Whitewater real estate deal, according to a May 1999 GAO report.

Starr's investigation turned over documentation to the GOP-led House Judiciary Committee, which impeached Bill Clinton in December 1998 on single charges of perjury and obstruction of justice related to his affair with intern Monica Lewinsky. A Senate vote in February 1999 failed to convict Clinton on those charges.

So far, Mueller's team has charged four people as part of the ongoing investigation.

So far, Mueller's team has charged four people as part of the ongoing investigation

Last week, Flynn pleaded guilty to a felony charge of lying to the FBI and agreed to cooperate with Mueller's investigators. George Papadopoulos, a former Trump campaign foreign policy adviser, made a similar plea deal with Mueller's prosecutors in October. Both men admitted to lying to the FBI about their contacts with Russians related to their work for Trump.

In addition, Manafort and his longtime business associate, Rick Gates, are currently facing several felony charges brought by the special counsel's office. Those charges involve allegations of money laundering and other financial crimes related to their political consulting work in Ukraine.

In its report, Mueller's team notes that it draws its funding from a revolving 'permanent, indefinite' appropriation for independent counsels provided by Congress, as well as from non-reimbursed Justice Department agency expenditures and from intelligence agency appropriations.

In a statement, the special counsel's office said it will release a similar expenditure report after March 31, 2018.