Veterans advocates say they are frustrated at the lack of transparency around the Department of Veterans Affairs' use of the unproven drug hydroxychloroquine as a treatment for the coronavirus.

After the release of a preliminary study of veterans hospitalized with COVID-19 last week that showed that hydroxychloroquine — an anti-malaria drug touted by President Donald Trump — had no benefit and caused a greater rate of deaths, the groups want answers and are worried that they may have been misled by the agency on its recent purchase of the drug.

Jeremy Butler, CEO of Iraq and Afghanistan Veterans of America, said that the results of the study were "incredibly troubling for a number of reasons" and that the VA needed to provide answers.

"Why were veterans who were receiving treatment from a federal agency being treated with an unproven and speculative drug?" he asked in a statement. "What was the approval process used by doctors, patients and their families in discussing and agreeing upon this treatment option? At what point did the VA know that the results were this dire and when did they act upon those results? What are the VA's current procedures for approving and administering the drug?"

The study, which included results from 368 patients, is the largest examination of the drug's effect on patients suffering from COVID-19, the disease caused by the coronavirus. Researchers concluded that there was a greater prevalence of death among those who took the drug compared to those who received only standard care.

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VA Secretary Robert Wilkie briefly mentioned the study in an interview with MSNBC last week, downplaying its results.

"That's an observational study," he said. "It's not a clinical study. It was done on a small number of veterans — sadly, those of whom were in the last stages of life, and the drug was given to them. And I have to also say that we know the drug has been working on middle-age and younger veterans."

A VA spokeswoman also pushed back on the idea that the agency was testing the drug on patients even though it has not been scientifically evaluated for its uses to treat COVID-19.

"VA is not testing hydroxychloroquine," said Christine Noel, the agency's press secretary. "It is using it to treat COVID-19 in cases where Veteran patients and their providers determine it is medically necessary, and in a manner consistent with current FDA guidance."

Noel said that the study was not a clinical trial, calling it "an analysis of retrospective data regarding hospitalized patients," and that the drug was "provided to VA's sickest COVID-19 patients, many times as a last resort."

More than 370 veterans have died of the coronavirus, and about 5,800 are confirmed carriers, according to the agency's numbers Wednesday.

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Butler told NBC News that the drug and the study were briefly mentioned in two weekly meetings that veterans service organizations like his have had with agency leaders, including Wilkie.

Multiple attendees of the meetings said they are not open discussions with the organizations. Instead, they said, the VA says how it is handling various challenges, and then leaders take a few questions curated ahead of time.

Butler said the VA first characterized a large purchase of the drug as having been made for its patients who suffer from lupus or arthritis — conditions the drug has been scientifically proven capable of treating. At the most recent meeting Wednesday, leadership briefly touched on the study but quickly moved on, he said.

"In our notes from a week ago, it appeared Wilkie said that they are standing by for medical guidance on hydroxychloroquine," he said. "We took that as meaning they were not using it."

Sen. Jon Tester of Montana, the ranking Democrat on the Veteran Affairs Committee, sent questions to the VA about its large order of hydroxychloroquine after the study was released.

"After this order was put in, we asked about the purpose of this order, and we were told it was for routine treatment of lupus and arthritis," Tester said in his request for information.

Tester also asked whether the patients were being treated after having provided informed consent, what guidance the VA provided to facilities using the drug and whether it was engaged in any further studies.

As of late last week, his office was still waiting for the VA to respond.

The Veterans of Foreign Wars, one of the largest veterans service organizations in the country and one that represents many elderly veterans, said it was very concerned about the study and had many questions about the plan behind it and how it was rolled out.

Terrence Hayes, the VFW's director of communications, met with the VA leadership twice last week, and he said he has received few answers and heard little about it.

"It feels like they've talked about everything but [the study]," he said.

It is not just advocacy groups that are upset that veterans were used in a study for a drug that has only anecdotal support within the medical community, which Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, has warned could be a false hope.

Rep. Mark Takano, D-Calif., chairman of the Veterans Affairs Committee, said that while the results may be preliminary, he found the data released by the VA to be concerning, saying it showed that it "may be premature to treat veterans" with the drug, "particularly in light of NIH recommending these drugs not be used." NIH is the National Institutes of Health.

"I'm not convinced we're ready for widespread off-label use of hydroxychloroquine at VA — especially when the administration hasn't done enough research on its safety and efficacy for treating COVID-19," he added. "When it comes to treating our veterans, we must rely on expert opinion and the proven science that leads to consistent guidance across the country."

Rep. Ruben Gallego of Arizona, an Iraq War veteran who is Democratic member of Takano's committee, went further, saying he was outraged that the VA appeared to turn veterans into experimental subjects to support an unproven treatment touted by the president.

"I think the most important thing is we need to keep veterans safe, not necessarily being test labs for the president to score political points," Gallego said.

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As the pandemic has spread across the U.S., veterans service organizations have said numerous times that the country's second-largest government agency, charged with caring for the nation's veterans population, has remained tight-lipped in its response to the spread of the coronavirus.

Many say they are concerned about reports that some veterans hospitals across the country have run short of personal protective equipment, even as the VA continues to say it has enough in stock.

The lack of specificity and media availability by the department's leadership has some concerned.

"The largest health care provider in the country and the backstop for the civilian health care system isn't out front being heard from and having questions asked of it on a daily basis," Butler said. "From the basic standpoint of a democratic society facing one of the largest health crises in a century, that seems incorrect and inappropriate."

CORRECTION (April 26, 2020, 7 p.m. ET): An earlier version of this article misspelled the first name of the communications director for the Veterans of Foreign Wars. He is Terrence Hayes, not Terrance.