The Environmental Protection Agency's inspector general on Friday said it plans to expand its investigation of Administrator Scott Pruitt's travel.

The office will now examine all of Pruitt's travel through Sept. 30, not just his frequent travel to and from his home state of Oklahoma.

Recent disclosures have shown that Pruitt has taken at least four noncommercial and military flights since mid-February, at a cost of more than $58,000.

"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 from the inspector general's office.

The EPA's inspector general had announced in August it opened a probe into Pruitt's travel to Oklahoma, where he served as Republican attorney general, because of congressional requests and a complaint filed by the nonprofit Environment Integrity Project in July.

The nonprofit released a report that documented Pruitt's trips using records acquired through Freedom of Information Act requests.

At the time, Pruitt's trips home had cost $12,000, and he spent 43 out of 92 days from March through May in Oklahoma or traveling to or from the state, according to the group.

Pruitt is one of many Cabinet members to face scrutiny over their travel habits.

Health and Human Services Secretary Tom Price resigned a week ago for spending close to $1 million on private air travel.

The Interior Department's inspector general's office recently opened an investigation into Secretary Ryan Zinke's flights, which involved travel to talk to sports teams owned by campaign contributors.

And Reuters reported Wednesday that Energy Secretary Rick Perry took a private chartered flight to visit a uranium-processing facility in Ohio the day before Price resigned.