Documents show he has blown through overtime budgets and diverted officers away from investigating environmental crimes, in order to give himself added protection.
Mr Pruitt who was picked by Donald Trump to head up the Environment Protection Agency (EPA), faces a growing list of potential ethics scandals, including the revelation two aides were granted huge pay rises against White House direction.
Shortly after taking the reins, last February, Mr Pruitt demoted the career staff member heading his security detail and replaced him with EPA senior special agent Pasquale "Nino" Perrotta, a former Secret Service agent who operates a private security company.
An EPA official with direct knowledge of Mr Pruitt's security spending told the Associated Press news agency that Mr Perrotta oversaw a rapid expansion of Mr Pruitt's security detail.
Guarding him day and night, they went with the former Oklahoma attorney general on family holidays and when he returned to home state. The EPA official spoke on condition of anonymity for fear of retaliation.
Mr Perrotta also signed off on new procedures that allowed Mr Pruitt, who sued the Environment Protection Agency 14 times in his previous role, to fly first-class on commercial airliners.
The security chief typically sat next to him with other security staff farther back in the plane. Mr Pruitt's premium status gave him and his security chief access to VIP airport lounges.
The EPA official said there are legitimate concerns about Mr Pruitt's safety, given public opposition to his rollbacks of anti-pollution measures.
But Mr Pruitt's ambitious domestic and international travel led to rapidly escalating costs, with the security detail racking up so much overtime that many hit annual salary caps of about $160,000 ($113,000).
The demands of providing 24-hour coverage meant some investigators were taken away from field work, including one occasion when Mr Pruitt travelled to California for a family holiday.
The EPA official said total security costs approached $3m (£2.12m) when pay is added to travel expenses.
EPA spokesman Jahan Wilcox said that Mr Pruitt has faced an "unprecedented" amount of death threats against him and his family.
"Americans should all agree that members of the president's cabinet should be kept safe from these violent threats," Mr Wilcox said.
A nationwide search of state and federal court records by the AP found no case where anyone has been arrested or charged with threatening Mr Pruitt. EPA's press office did not respond when asked to provide details of any specific threats or arrests.
Mr Pruitt has said his use of first-class airfare was initiated following unpleasant interactions with other travellers. In one incident, someone yelled a profanity as he walked through the airport.
The EPA administrator has come under intense scrutiny for ethics issues and outsized spending. Among the concerns: massive raises for two of closest aides and his rental of a Capitol Hill apartment tied to a lobbyist who represents fossil fuel clients.
At least three congressional Republicans and a chorus of Democrats have called for Mr Pruitt to resign or be fired. But Donald Trump is standing by him for the moment.
A review of Mr Pruitt's ethical conduct by White House officials is underway, adding to investigations by congressional oversight committees and EPA's inspector general.
The 49-year-old was closely aligned with the oil and gas industry as Oklahoma's state attorney general before being tapped by Mr Trump. The US president has praised Mr Pruitt's relentless efforts to scrap, delay or rewrite Obama-era environmental regulations. He has also championed budget cuts and staff reductions at the agency so deep that even Republican budget hawks in Congress refused to implement them.
EPA's press office has refused to disclose the cost of Mr Pruitt's security or the size of his security detail, saying doing so could imperil his personal safety.
But other sources within EPA and documents released through public information requests help provide a window into the ballooning costs.
In his first three months in office, before pricey overseas trips to Italy and Morocco, the price tag for Mr Pruitt's security detail hit more than $832,000 (£590,000), according to EPA documents released through a public information request.
Nearly three dozen EPA security and law enforcement agents were assigned to Mr Pruitt, according to a summary of six weeks of weekly schedules obtained by Democratic senator Sheldon Whitehouse of Rhode Island.
Those schedules show multiple EPA security agents accompanied Mr Pruitt on a family vacation to California that featured a day at Disneyland and a New Year's Day football game where his home state Oklahoma Sooners were playing in the Rose Bowl. Multiple agents also accompanied Mr Pruitt to a baseball game at the University of Kentucky and at his house outside Tulsa, during which no official EPA events were scheduled.
On weekend trips home for Sooners football games, when taxpayers weren't paying for his ticket, the EPA official said Mr Pruitt flew coach. He sometimes used a companion pass obtained with frequent flyer miles accumulated by Ken Wagner, a former law partner whom Mr Pruitt hired as a senior adviser at EPA at a salary of more than $172,000 (£122,000). Taxpayers still covered the airfare for the administrator's security detail.
Mr Pruitt's predecessor, Gina McCarthy, had a security detail that numbered about a half dozen, less than a third the size of Pruitt's. She flew coach and was not accompanied by security during her off hours, like on weekend trips home to Boston.
Mr Pruitt was accompanied by nine aides and a security detail during a trip to Italy in June that cost more than $120,000 (£85,000). He visited the US embassy in Rome and took a private tour of the Vatican before briefly attending a meeting of G7 environmental ministers in Bologna.
Private Italian security guards hired by Mr Perrotta helped arrange an expansive motorcade for Mr Pruitt and his entourage, according to the EPA official with direct knowledge of the trip. The source described the Italian additions as personal friends of Mr Perrotta, who joined Mr Pruitt and his EPA staff for an hours-long dinner at an upscale restaurant.
Mr Perrotta's biography, on the website of his company, Sequoia Security Group, says that during his earlier stint with the Secret Service he worked with the Guardia di Finanza, the Italian finance police.
The EPA spent nearly $9,000 (£6,389) last year on increased counter-surveillance precautions for Mr Pruitt, including hiring a private contractor to sweep his office for hidden listening devices and installing sophisticated biometric locks for the doors. The payment for the bug sweep went to a vice president at Mr Perrotta's security company.
The EPA official who spoke to AP said Mr Perrotta also arranged the installation of a $43,000 (£30,000) soundproof phone booth for Mr Pruitt's office.
At least five EPA officials were placed on leave, reassigned or demoted after pushing back against spending requests such as a $100,000 (£70,000)-a-month private jet membership, a bulletproof vehicle and $70,000 (£50,000) for furniture such as a bulletproof desk for the armed security officer always stationed inside the administrator's office suite.
Those purchases were not approved. But Mr Pruitt got an ornate refurbished desk comparable in grandeur to the one in the Oval Office.
Among the officials who faced consequences for resisting such spending was EPA deputy chief of staff for operations Kevin Chmielewski, a former Trump campaign staffer who was placed on unpaid administrative leave this year.
The prior head of Mr Pruitt's security detail, Eric Weese, was demoted last year after he refused Mr Pruitt's demand to use the lights and sirens on his government-owned SUV to get him through Washington traffic to the airport and dinner reservations.
Additional reporting by AP
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