WASHINGTON -- For EPA Administrator Scott Pruitt, June 7 was a busy day.

First, he had a trip to Cincinnati with President Trump. Then, he went on to New York's JFK Airport on an Air Force jet, leaving behind $350 commercial flights and sticking taxpayers with a bill for at least $20,000.

He then flew to Italy for an international summit that didn't start until three days later, and he left that meeting a day early. It's unclear why he was in a rush.

Separately, Pruitt's frequent flights to his home state of Oklahoma have attracted the attention of the EPA's inspector general.

EPA Administrator Scott Pruitt Reuters

It's not just Pruitt. At the Treasury Department, the inspector general is looking at Secretary Steve Mnuchin's use of military aircraft. And the Health and Human Services inspector general is reviewing Secretary Tom Price's chartered flights that cost $400,000.

"They're conducting both an internal and an IG review, and all travels on private charter has been suspended until that's completed," White House press secretary Sarah Sanders said in response to questions about the officials' travel.

In addition to boarding that more expensive military aircraft in June, CBS News has learned Pruitt took a private plane from Denver to Durango, Colorado, and back on August 4 for a meeting that included state officials.

An EPA spokesperson told CBS News that Pruitt chartered the plane "after his flight was significantly delayed in order to ensure that he did not miss a critical meeting," and ethics officials were consulted.

The Colorado governor's office offered Pruitt a ride on his government aircraft, but the EPA declined.

Eric Schaeffer CBS News

"There needs to be an explanation of what that cost was and why it was necessary," said Eric Schaeffer, a former director of the EPA's Office of Civil Enforcement who now runs the Environmental Integrity Project.

"Especially when the budget is shrinking for your agency, the expectation is that you'll travel as economically as possible. Generally that does not include chartering private jets for your travel," Schaeffer said.

CBS News called and emailed the EPA half a dozen times. We wanted to know why it was necessary to use a military jet, and also why they turned down the invitation to fly with Colorado's governor.

An EPA spokesperson declined to comment.