Compared with his predecessors, Pruitt's security costs are significantly higher.
Between 2009 and 2016, spending ranged from $1.6 million to $2 million annually for the two previous administrators' security.
Patrick Sullivan, the EPA's Assistant Inspector General for Investigations, claimed that Pruitt faced significant more threats than the previous administrator, Gina McCarthy.
The AP previously reported that the EPA spent nearly $3 million dollars in taxpayer money on an around-the-clock, 20-member security detail that was also significantly larger than his predecessors' detail.
"Administrator Pruitt has faced an unprecedented amount of death threats against him and to provide transparency EPA will post the costs of his security detail and pro-actively release these numbers on a quarterly basis. Americans should all agree that members of the president's cabinet should be kept safe from violent threats," EPA spokesman Jahan Wilcox said.
CBS News reported in April that Pruitt received tweets threatening to "put a bullet between your eyes" and another wishing him "a very painful and horrible death through poisoning."
Pruitt's daughter also received a menacing message on Facebook, the document obtained by CBS News revealed.
The administrator has also drawn criticism for flying first class during his first year in office, an expense that contributed to the substantial overall figure.
During an interview in February with CBS News White House correspondent Major Garrett, Pruitt said that he would start flying coach, depending on the nature of various security threats.
"There's a change coming, because look the security threat matters," Pruitt said. "What I've told them going forward is this: There is a change occurring, you're going to accommodate the security threats as they exist, you're going to accommodate those in all ways, alternate ways, up to and including flying coach, and that is what's going to happen on my very next flight. So those things are happening right away."
Pruitt's expenditures have remained the subject of intense scrutiny. Yet the embattled cabinet secretary has remained in Mr. Trump's good graces despite being the target of at least 10 investigations into various potential ethical violations.
The EPA's Office of Inspector General could not comment on the total number of EPA OIG audits, but it has made at least five investigations publicly known and has received seven additional requests to investigate various other issues that have been folded "into ongoing audits and/or will start the review once resources become available."
Investigations into Pruitt's travel, protective service detail, emails, and "authority to fill administratively determined positions" pay are ongoing, according to Kentia Elbaum, a spokesperson for the EPA inspector general.
