that plane - $15,000 every time it takes off. 

But just like it wasn't up to Wray then, Patel doesn't have a choice in how he travels.

A post-9/11 government regulation requires the attorney general, and since 2011, the FBI director, to use government aircraft for all travel, including personal trips. The reason for the policy, according to the Justice Department, is that the president and other top officials must always have secure communications available with the FBI director and attorney general, in case of national security or other emergencies.

Grassley gave Patel until May 19 to respond to the voluntary request.

In the letter, Grassley, a Republican, points to other times he has probed an FBI director's spending habits.

 For decades, regardless of which political party is in the White House, I have worked to ensure that taxpayers dollars are protected from waste, fraud, and abuse  Grassley wrote.

CNN has reached out to Grassley's office for comment.

Grassley's letter comes as House and Senate Democrats have launched their own probe into Patel's spending, including references to whistleblower accounts.