Costly cabinet travel prompts senators to ask federal budget office to explain spending

The inquiries come amid probes into cabinet officials' travel and other spending

Several members of Trump's cabinet have been under scrutiny for the cost of their travel and other expensive purchases. The president asked former Health and Human Services Secretary Tom Price to resign last September after reports that his travel on private planes cost more than $1 million of taxpayer money.

"It is our duty to provide oversight and ensure that taxpayer money is spent responsibly, without waste, fraud, or abuse," the senators wrote in the letter, which was reviewed by ABC News. "As you have said yourself about past issues with Administration travel, “just because something is legal doesn’t make it right.” We owe it to the American people to ensure with the utmost vigilance that tax dollars are being spent appropriately."

The senators are also asking Mulvaney for documents on all travel approved by the White House Chief of Staff related to travel on government-owned or charter planes. OMB said in a memo issued after Price resigned that some travel would need to be approved by the chief of staff and that the chief of staff would provide further guidance on the use of government and private planes.

Federal law says that government officials should use the least expensive travel option available and should use commercial flights, unless otherwise authorized.