Environmental Protection Agency Administrator Scott Pruitt speaks with coal miners at the Harvey Mine on April 13 in Sycamore, Pa. (Justin Merriman/Getty Images)

The Environmental Protection Agency’s inspector general acknowledged plans Friday to expand its inquiry into Administrator Scott Pruitt’s travel habits, marking the latest Trump Cabinet member to face scrutiny from his own agency for taxpayer-funded trips.

The move came after recent disclosures that Pruitt had taken at least four noncommercial and military flights since mid-February, costing taxpayers more than $58,000 to fly him to various parts of the country, according to records provided to a congressional oversight committee and obtained by The Washington Post.

The EPA inspector general’s office announced in August that it had opened an inquiry into Pruitt’s frequent travel to his home state of Oklahoma. The internal watchdog at the time said its investigation was triggered by “congressional requests and a hotline complaint, all of which expressed concerns about Administrator Pruitt’s travel — primarily his frequent travel to and from his home state of Oklahoma at taxpayer expense.”

The probe was triggered in part by findings from the Environmental Integrity Project, a nonprofit group that detailed through public records that Pruitt had spent nearly half of the days in March, April and May in Oklahoma. Initially, EPA investigators said they planned to audit Pruitt’s travel records, as well as those of his security and top aides, through the end of July.

But on Friday, the inspector general’s office said it would expand that inquiry to include all of Pruitt’s travel through the end of September, and not just trips to Oklahoma.

“We will review supporting documentation and conduct interviews with management and staff to determine whether the EPA followed applicable policies and practices, and complied with federal requirements,” states Friday’s letter, a copy of which was sent to Pruitt.

The EPA has insisted that Pruitt flies commercially when feasible and that all the trips in question were approved by officials in the agency’s Office of General Counsel.

“The Trump Administration will work to ensure all officials follow appropriate rules and regulations when traveling, including seeking commercial options at all times appropriate and feasible, to ensure the efficient use of government resources,” EPA spokeswoman Liz Bowman said in a statement Friday.

The most expensive of Pruitt’s four noncommercial trips came in early June, when Pruitt traveled from Andrews Air Force Base to Cincinnati to join President Trump as he pitched a plan to revamp U.S. infrastructure. From there, Pruitt and several staff members continued on a military jet to John F. Kennedy airport in New York to catch a flight to Italy for an international meeting of environmental ministers. The cost of that flight was $36,068.50.

The EPA has argued that “no viable commercial flights” would have allowed Pruitt to make his plane to Italy, where he had “scheduled meetings with Vatican officials the next day.”

Weeks later, on July 27, Pruitt and a half-dozen staff members arranged a flight on an Interior Department plane from Tulsa to the tiny outpost of Guymon, Okla., at a cost of $14,434.50. The EPA noted that “time constraints” on Pruitt’s schedule wouldn’t allow him to make the 10-hour round-trip drive. The purpose of the trip was to meet with landowners “whose farms have been affected” by a controversial rule regulating water bodies in the United States, according to the agency.

On another occasion, Pruitt and three staff members arranged a private air charter on Aug. 4, on a trip from Denver to Durango, Colo. The flight cost $5,719.58. According to the EPA, the commercial flight Pruitt had planned to take “was delayed ultimately for eight hours, which would have caused him to miss a mission critical meeting at Gold King Mine” with Gov. John Hickenlooper (D) and other officials. Pruitt that day criticized the EPA’s previous handling of the mine, where the agency accidentally triggered a spill two years earlier.

Sen. Sheldon Whitehouse (D-R.I.) wrote to the EPA inspector general on Sept. 26, requesting an additional inquiry into Pruitt’s travels. He cited the “troubling pattern” of Pruitt and other Cabinet officials taking private and military flights at taxpayer expense, arguing that the use of noncommercial flights “warrants an expansion of your investigation beyond its current scope.”

In a statement Friday, Whitehouse welcomed the widening of the probe, noting that Pruitt has proposed cutting the agency by 30 percent even as he has “spent tens of thousands of taxpayers dollars on chartered flights and a secure communication chamber in his office, even when the agency already has one.”

“I’m glad to see the Inspector General has expanded this investigation into Administrator Pruitt’s excessive and dubious spending,” Whitehouse added. “The American people deserve to know whether these expenses are a good use of their money.” ‎

Juliet Eilperin contributed to this report.

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