The agency violated the law by using more than $5,000 of fiscal 2017 appropriated funds for an unintended objective without giving lawmakers advance notice, GAO investigators wrote in a report released Monday.
GAO disagreed with EPA's interpretation of the law.
According to EPA, the booth "not only enables the Administrator to make and receive phone calls to discuss sensitive information, but it also enables him to use this area to make and receive classified telephone calls (up to the top secret level) for the goal of conducting agency business".
Democrats and a few Republicans have been calling on Pruitt either to resign or be fired since late last month, when media reports first disclosed that he had rented a Capitol Hill condo tied to an oil and gas lobbyist at the bargain rate of $50 a night.
But Armstrong said the "EPA was required to notify appropriations committees of its proposed obligation".
The EPA also said that the booth was "analogous to other functional items an employee might require to perform his job duties such as a high speed computer, high speed copier/scanner, or television" - even though no previous agency head had a similar set-up and there were secure lines already in place at EPA headquarters. It also released federal invoices under a Freedom of Information Act request by the left-leaning group American Oversight showing the agency hired one contractor to pour a 55-square-foot concrete block that was more than two-feet thick, at a cost of $3,470, another to install a drop ceiling for $3,361, and a third to patch and paint the room for $3,350.
The GAO, a government spending watchdog that reports to Congress, cites a requirement that executive branch agencies must notify Congress before spending more than $5,000 on certain office improvements.
Trump has rebuffed recent calls by both Democratic and Republican lawmakers to fire Pruitt, saying he is doing a "fantastic job" and is well-loved in "coal and energy country".
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Kevin Chmielewski, formerly Pruitt's deputy chief of staff for operations, told congressional investigators last week that although Jackson signed off on the raises, Pruitt knew about the entire process.
"We draw no conclusions regarding whether the installation of the privacy booth was the only, or the best, way for EPA to provide a secure telephone line for the administrator", GAO said.
EPA Chief of Workers Ryan Jackson. But his chief of staff appears to have signed the forms "for Scott Pruitt", suggesting that he was acting with explicit or implicit permission.
At least four members of Congress, including Senator Tom Udall (D, New Mexico), accused Pruitt of blatantly abusing his position to enjoy unnecessary perks at his office.
"An illegal privacy booth to conduct secret discussions with his polluter friends does nothing to help our health or environment", Udall said. Donald Trump shouldn't wait to see what ethical norm or law Pruitt breaks next.
"It is critical that EPA and all federal agencies comply with notification requirements to Congress before spending taxpayer dollars". "EPA must give a full public accounting of this expenditure and explain why the agency thinks it was complying with the law".