Mueller's spokesman Peter Carr said the latest accounting is the first "with the office operating with full staff, travel, rent, communications and IT services."

Special counsel Robert Mueller 's travel costs are up almost 80 percent, according to a report his office issued Thursday, a sign that the investigation added more destinations to its sprawling inquiry in the six months between last October and March of this year. Mueller, who's investigating President Donald Trump 's ties to Russia, doesn't disclose where his staff travels. But there is no shortage of locations that have garnered the interest of investigators in what has become a global investigation into the financial dealings of the president's associates. Travel costs totaled $532,340 in the six months covered in Mueller's latest, and second, expenditure report. That amounts to about $90,000 per month, compared with about $50,000 per month in the previous period.

"The prior statement represented a time when the office was getting started, and it did not reach full operations in these areas until the latter half of that reporting period," Carr said.

Investigators appeared to have escalated their interest in Trump's personal lawyer and fixer Michael Cohen in the six-month period.

According to a report in McClatchy, Mueller's agents interviewed a Trump business associate involved in The Trump Organization's overseas deals about Cohen during the first week of April, right after the end of the reporting period.



Cohen, according to McClatchy, was personally involved in Trump Organization deals secured in Georgia, Kazakhstan and Russia.

Also in April, Mueller reportedly secured evidence that Cohen had traveled to Prague in 2016, a claim that Cohen had previously denied. Cohen's Prague travel was an element in the dossier compiled by British former spy Christopher Steele regarding Trump's connections to Russia.

FBI investigators raided Cohen's office and hotel room April 9 after receiving a referral from the special counsel's office.

Separately, in February, a prosecutor in Ukraine reportedly reached out to Mueller to offer to cooperate with the investigation into Paul Manafort, Trump's campaign chairman who Mueller indicted in October 2017 for failing to register as a foreign agent. A grand jury returned a superseding indictment in February of this year.

By April, the prosecutor, who told The New York Times that he did not receive a response to his offer, said that he had lost the authority to investigate Manafort, and so could no longer cooperate with Mueller.

Overall, Mueller's expenditures rose only 5 percent in the latest expenditure report, to about $750,000 a month. The total for the six-month period was about $4.5 million.

While items such as rent, compensation and transportation rose sharply, there was a 95 percent decrease in the category "acquisition of equipment." The cost of supplies and materials also declined.

— CNBC's Christina Wilkie contributed to this report.