Special Counsel Robert Mueller has spent more than $3.2 million in the opening months of his federal criminal investigation into Russian interference in last year’s U.S. election and whether President Donald Trump or anyone close to him colluded in it.

The spending, compiled by Mueller’s office, was released by the Justice Department on Tuesday and covers the period from May 17 through Sept. 30.

Funds were spent on personnel, travel and office space, the Justice Department said.

Although limited in detail, the spending report offers the first insight into the scope and scale of Mueller’s investigation, which has resulted so far in criminal charges against four people who worked on Trump’s presidential campaign.

Mueller’s spending “is entirely reasonable given the results we’ve already seen,” Senator Dianne Feinstein of California, the top Democrat on the Judiciary Committee which is conducting its own investigation, said in a statement. “With two individuals having entered guilty pleas and two more facing federal charges, it’s clear the investigation is moving forward.”

It’s difficult to make comparisons to other investigations led by specially designated prosecutors or to draw conclusions about how much Mueller will ultimately spend.

For example, Patrick Fitzgerald was a special counsel who investigated a far narrower subject: a leak that exposed the identity of covert CIA officer Valerie Plame. He spent $26,592 in the probe’s first phase, from the time he was appointed on Dec. 30, 2003 to March 31, 2004, on personnel, travel, contracts and supplies, according to an audit from the Government Accountability Office.

An investigation into President Bill Clinton by independent counsel Kenneth Starr cost about $40 million from Jan. 24, 1994 to Sept. 30, 1998, according to the GAO. Starr’s sprawling investigation, which led the House to impeach Clinton for perjury, contributed to Congress letting a law that gave special investigators wide independence expire in 1999.

Since then, special counsels have been governed by a Justice Department regulation. 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, a former FBI director, appears to be moving at a faster pace than previous investigators. In October, the first indictments in his probe were unsealed in court against Trump’s former campaign manager Paul Manafort and an aide, Rick Gates. Mueller also disclosed that he secured a guilty plea from George Papadopoulos, who was a junior foreign-policy adviser to the Trump campaign, for lying to the FBI about contacts with Russian operatives.

On Friday, Mueller revealed a guilty plea by Michael Flynn, Trump’s former national security adviser, who admitted lying to FBI agents.