An Army Colonel who blew the whistle on a task force that was responsible for building a $43 million gas station in Afghanistan says he is being retaliated against, according to a prominent U.S. senator.

Army Col. John Hope was the director of operations for the $800 million task force when he questioned its lack of accountability, and then said he had “been singled out for retaliation and retribution” for “speaking truth,” according to a letter Sen. Chuck Grassley (R-Iowa) sent to Defense Secretary Ashton Carter last week.

The Special Inspector General for Afghanistan Reconstruction recently released a scathing report on one of the task force’s programs — the $43 million compressed natural gas station, which the IG said should have cost closer to $500,000.

Hope told Grassley’s office that after he spoke out about the problems of the task force, which is now defunct, his performance review was delayed, jeopardizing his career.

“If the Pentagon is retaliating against someone for speaking out on poor accountability and wasteful spending, that’s unacceptable,” Grassley said in a statement.  “It’s detrimental to the individual and to the taxpayers.”

The Pentagon and Hope did not immediately respond to requests for comment.

The high cost of the gas station has angered many in Congress. Sen. Kelly Ayotte (R-N.H.) has scheduled a hearing on it for next month. And Sen. Claire McCaskill (D-Mo.) said it was one of the worst cases of wasteful spending that she has ever seen.

“There are few things in this job that literally make my jaw drop,” she said in a statement. “But of all the examples of wasteful projects in Iraq and Afghanistan that the Pentagon began prior to our wartime contracting reforms, this genuinely shocked me.”

The gas station was intended to help Afghanistan curb its dependence on foreign petroleum products and take advantage of domestic energy. But investigators found that Afghanistan does not have the natural gas transmission infrastructure to support a “viable market” for cars that used compressed natural gas.

And the cost of converting gasoline-powered cars to run on natural gas “may be prohibitive for the average Afghan.” The cost to do so is estimated at about $700 per car, while the average annual income in Afghanistan is $690.

“In sum, it is not clear why [the task force] believed the CNG filling station should be undertaken,” wrote John Sopko, the IG.

“One of the most troubling aspects of this project is that the Department of Defense claims that it is unable to provide an explanation for the high cost of the project or to answer any other questions concerning its planning, implementation or outcome,” he wrote.

The department that was in charge of it, the Task Force for Stability and Business Operations, has closed, and so the Pentagon said it couldn’t comment on its activities, Sopko’s letter said.

Grassley said he plans to seek an audit of the task force’s expenditures and will ask fellow senators to join the request.