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.

"Because EPA violated this statutory requirement, we also conclude that EPA violated the Antideficiency Act and should report its violation to the Congress and the President as required by law," GAO said in a statement by one of its attorneys, Julie Matta.

It is not yet clear what the consequences, if any, may be. President Donald Trump has repeatedly backed Pruitt through the revelations -- about expensive travel, a below-market-rate lease from an energy lobbyist, and raises given to aides against White House direction -- calling him a "good man" who "is doing a great job!"

Pruitt is the first EPA administrator to employ an around-the-clock security detail. He has flown first or business class when traveling. He installed biometric locks on his office and closed the area around it to EPA personnel who are not on an approved list. The agency paid $3,000 to a business partner of his security chief to check his office for spying technology.

The EPA disputed that it was required to notify Congress before making the purchases and did nothing wrong. EPA spokeswoman Liz Bowman said the agency "will be sending Congress the necessary information this week."

The $24,570 "privacy booth for the administrator" was ordered in August from a Virginia-based company that specializing in soundproofing materials. The total price of around $43,000 included renovations to prepare space for the booth, including removing a closed circuit camera system.

The room, known elsewhere in government as a Sensitive Compartmented Information Facility or SCIF, is typically used for reading and meetings involving classified information. The EPA headquarters building already had a SCIF, according to former employees.

Pruitt told Congress in December he uses the booth for communications with the White House. EPA also told the GAO 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 purpose of conducting agency business."

EPA's "Security Management Division requires that a classified telephone must be located in an area where the employee can have private conversations. That is, a classified phone cannot simply be put on an office desk or in a conference room," the EPA told GAO in a March 23 letter that the agency released on Monday.

EPA interpreted the requirement it notify Congress applies to "aesthetic improvement(s)" made to Pruitt's office, and compared the $43,000 booth to purchasing other electronics required for an employee's job.

"The privacy booth is analogous to other functional functional items an employee might require to perform his job duties such as a high speed computer, high speed copier/scanner, or television," the EPA letter said.

The GAO found the law is more broad and applies to changes like the booth, which provides "a new, practical addition to the office." The eight-page report spends several paragraphs contemplating the meanings of words like "furnish" and "redecorate."

Last week, a former deputy chief of staff to Pruitt, spoke to congressional investigators about his concerns with Pruitt's spending. After the meetings, several Democratic lawmakers raised concerns that a variety of Pruitt's actions are "unethical and potentially illegal."