Washington (CNN) The US Air Force is facing more questions as to why it spent tens of thousands of dollars over the last three years on large cups that can reheat beverages, like coffee or tea, on refueling tankers and cargo aircraft during flight.

Despite being assured by Air Force Secretary Dr. Heather Wilson earlier this month that the service has "suspended its purchasing of the exorbitantly priced cups," Sen. Chuck Grassley sent a follow-up letter last week asking for further explanation as to why they were purchased in the first place.

"In my previous letter, I asked what cheaper alternatives had been explored, and if this water-heating cup was truly necessary. In your reply, you indicated that the Air Force is exploring ways to use 3D printing and other technology to cut costs on spare parts, and conducting a review of procurement to identify overpriced items. Your response does not indicate if alternatives to this specific water-heating cup have been explored or discovered," the Iowa Republican wrote in his latest inquiry.

In a statement to CNN, the Air Force also confirmed it is no longer buying the reheating cups "used in large transport aircraft as they work more cost-effective solutions."

Grassley raised the issue of the cups in an October 2 letter to Wilson after reports surfaced this summer that the 60th Ariel Port Squadron at Travis Air Force Base had spent $1,280 on each cup in 2018 -- a dramatic increase from the $693 per-cup price in 2016.

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