Senate Judiciary Chair Chuck Grassley (R-IA) on Thursday urged President Donald Trump to implore his Cabinet secretaries to stick to the most cost-effective modes of travel possible.

Grassley wrote a stern letter to the President amid a slew of reports revealing that members of Trump’s Cabinet have spent thousands on private and government planes to attend official events.

“Federal regulations specifically prohibit official travel by chartered jet when it is not the most cost-effective mode of travel ‘because the taxpayers should pay no more than necessary for your transportation,'” Grassley wrote in the letter to Trump. “Considering the many travel options to and from Washington, D.C., I’m urging you to emphasize to Cabinet secretaries the necessity of using reasonable and cost-effective modes of travel in accordance with federal restrictions.”

He noted that Health and Human Services Secretary Tom Price has spent more than $400,000 on private planes, and that both EPA Administrator Scott Pruitt and Treasury Secretary Steven Mnuchin are under investigation for their own use of non-commercial flights. Since Grassley sent his letter to Trump, it has also been revealed that Interior Secretary Ryan Zinke took private and government flights this year.

Price has said that he will pay the government back for his seats on the charter planes—not for the full cost of the flights—and he has halted his use of charter planes while the HHS inspector general reviews his travel.

Grassley urged Trump to issue a hold on non-commercial flights for all departments under review. Grassley also asked the President to share with the steps the administration has taken to make sure that Cabinet officials are using the most cost-effective modes of transportation possible.

Read the letter below: