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.

In her response to that letter, Wilson said the Air Force has spent $326,785 on nearly 400 cups since 2016 -- an average of $817.

"You are right to be concerned about the high costs of spare parts, and I remain thankful to have your support in addressing this problem," Wilson told the senator in a letter dated October 17.

The increasing cost of the cups was first reported by Fox News and Military.com in July as part of a report on how airmen at Travis Air Force Base are attempting to use 3D printing to develop a cost-effective way to replace the cups' plastic handles, which have a tendency to break.

"Unfortunately, when dropped, the handle breaks easily leading to the expenditure of several thousand dollars to replace the cup as replacement parts are not available," the Air Force said in a news release at the time.

Scrutiny over the pricey coffee cups has increased in recent months and the issue has drawn the ire of Grassley, who remains unsatisfied with the Air Force's explanation to date.

"I applaud the efforts by the Airmen at Travis Air Force Base who noticed wasteful spending, and took it upon themselves to come up with a better solution. ... I remain concerned that there are not adequate protocols in place to prevent reckless spending on parts from the start," he wrote in a letter to Wilson last week.

The Air Force maintains that it has been working to address the problem since it first came to light earlier this year.

"Our Airmen reported prices doubled on this out-of-production part and found a solution using 3D printing. The Air Force lauded the innovative solutions of Airmen at Travis AFB in June which was quickly followed by Congressional inquiries into how the prices of the electric heaters, purchased through the Defense Logistics Agency, had dramatically climbed," an Air Force spokesperson said in a statement regarding Grassley's latest inquiry.

"The Air Force believes there is potential to save millions of taxpayer dollars through 3D printing of parts for older aircraft as they drive changes to prevent being held hostage by obsolete supply chain practices," the statement said.