The Department of Defense wasted $43 million of U.S. taxpayer money on building a gas station in Afghanistan - 140 times more than the $500,000 it should have cost, according to a government watchdog report.

One of the most "troubling" issues, according to the report from the special inspector general for Afghanistan reconstruction (SIGAR), is that the Department of Defense has been unable or unwilling to even explain why the "ill-conceived" project cost so much money.

"Even considering security costs associated with construction and operation in Afghanistan, this level of expenditure appears gratuitous and extreme," SIGAR report, issued Monday, said.

"It's an outrageous waste of money that raises suspicions that there is something more there than just stupidity," John Sopko, the special inspector general, told NBC News. "There may be fraud. There may be corruption. But I cannot currently find out more about this because of the lack of cooperation."

One thing Sopko knows without a doubt is that the spending is "outrageous to the taxpayer," he told Fox News.

"I have never in my lifetime seen the Department of Defense or any government agency clam up and claim they don't know anything about a program," Sopko said. "Who's in charge? Why won't they talk? We have received more allegations about this program than we have received about any other program in Afghanistan."

The now-defunct Task Force for Stability and Business Operations (TFBSO) was tasked with overseeing the Downstream Gas Utilization project in Afghanistan, which was an attempt to wean the country off petroleum imports and revive its economy by utilizing its own vast natural gas reserves.

TFBSO was directed to build and operate Afghanistan's first compressed natural gas filling station, in the city of Sheberghan, and to also assist in developing the commercial market for domestic natural gas, NBC News explained.

While TFBSO succeeded in building the gas station, the group apparently failed to study the viability of the project or the obstacles it faced before construction began, and ended up wasting millions of dollars in the process.

"Although TFBSO achieved its immediate objective of building the CNG filling station, it apparently did so at an exorbitant cost to U.S. taxpayers," Sopko wrote to Secretary of Defense Ash Carter in a report, according to CNN. "In comparison, SIGAR found that a CNG filling station in Pakistan costs no more than $500,000 to construct."

SIGAR said that TFBSO should have conducted a feasibility study, which "might have noted that Afghanistan lacks the natural gas transmission and local distribution infrastructure necessary to support a viable market for CNG vehicles." Such a study would also have found that most Afghans would be unable to afford to convert their cars from gasoline to CNG.

Nonetheless, the task force awarded a $3 million contract to Central Asian Engineering to build the station, which became operational in May 2012.

In all, the task force spent $42,718,739 between 2011 and 2014 on construction and to supervise the initial operation of the station. That includes approximately $12.3 million in direct costs and $30 million in overhead costs, SIGAR said.

A similar CNG filling station "would have cost no more than $500,000 in neighboring Pakistan," SIGAR said.

"This is one of the worst examples of poor planning and just sheer stupidity," Sopko told NBC News, adding that he is "suspicious when I see something that cost 140 times more than it did and I find people trying to withhold or not cooperate with me."

Brian McKeon, the Department of Defense's principal deputy under secretary of defense for policy, told NBC News that the $43-million price tag sounded like something "wasn't done properly" and said the department is trying to cooperate with Sopko's investigation.

Between 2010 and 2014, Congress appropriated $820 million for the TFBSO, and SIGAR is currently conducting a broader investigation into its activities.

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