The Environmental Protection Agency's (EPA) internal watchdog is expanding its investigation into Administrator Scott Pruitt's travel habits.

The EPA's Office of Inspector General (OIG) said Friday that it will now examine Pruitt's taxpayer-funded travel through Sept. 30. It had previously limited the scope of its probe to Pruitt's travel to his home state of Oklahoma through July 31.

Pruitt has taken at least four charter or military flights, including one within Oklahoma, at a cost of more than $58,000. They included a flight from an event with President Trump in Cincinnati to New York City to catch a plane to Rome and flights to and from the Gold King Mine in Colorado.

The EPA has argued that each of Pruitt's noncommercial flights has been necessary and justified under the agency's guidelines.

The original inspector general probe stemmed from revelations from public records requests that Pruitt frequently uses federal funds in traveling to Oklahoma around weekends for work-related purposes, then stays in Tulsa - his home town - for the weekend before returning to Washington.

"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," the inspector general said in the notice.

It added a new wrinkle to the controversy surrounding spending at the EPA that Democrats and critics of the Trump administration say is wasteful, such as spending nearly $25,000 on a soundproof booth for Pruitt's office and a 24-hour security detail for the administrator that employs 18 officers.

The inspector general said in its Friday notice that the probe will continue to examine the cost and frequency of Pruitt's trips to Oklahoma, whether appropriate agency policies were followed and whether policies should change to prevent waste, fraud and abuse.

EPA officials have repeatedly defended Pruitt's travel to his home state.

"Administrator Pruitt is traveling the country to hear directly from the people impacted by EPA's regulations outside of the Washington bubble," the agency previously said. "This is nothing more than a distraction from the administrator's significant environmental accomplishments."

A handful of Trump's Cabinet secretaries have been under fire in recent days over noncommercial flights. Health and Human Services Secretary Tom Price resigned last week over his charter flights, which cost taxpayers hundreds of thousands of dollars.