During World War II, the U.S. military conducted secret chemical weapons experiments on approximately 4,000 American soldiers. Though the program was declassified in 1993, an ongoing investigation by NPR's Caitlin Dickerson has revealed that the Department of Veteran Affairs only located and offered compensation to 610 victims.

Now, NPR had released its own comprehensive, searchable database of the 3,900 veterans who were exposed to mustard gas and other chemical weapons, in an attempt to track down uncompensated survivors and their families.

Though chemical weapons have been used in warfare for at least 1,700 years, mustard gas is a modern invention. It first went into large-scale production during World War I. Depending on how the weapon is deployed, it can cause intense skin irritation, large fluid-filled blisters, bleeding and blistering in the respiratory system. Severe mustard agent burns are fatal, and those who recover face a chronic breathing problems higher risk of cancer.

In her NPR report last June, Dickerson explained the scope of the problem:

All of the World War II experiments with mustard gas were done in secret and weren't recorded on the subjects' official military records. Most do not have proof of what they went through. They received no follow-up health care or monitoring of any kind. And they were sworn to secrecy about the tests under threat of dishonorable discharge and military prison time, leaving some unable to receive adequate medical treatment for their injuries, because they couldn't tell doctors what happened to them.

"It felt like you were on fire," Rollins Edwards, now 93, told her. As an Army soldier, Edwards was exposed to chemical agents while standing inside a wooden gas chamber. "Guys started screaming and hollering and trying to break out. And then some of the guys fainted. And finally they opened the door and let us out, and the guys were just, they were in bad shape," he said.

The WWII experiments, which were conducted in Panama, were meant to determine how chemical weapons performed in tropical island climates. The military was searching for the "ideal chemical soldier" to resist potential attacks, according to medical historian Susan Smith. Experiments were often based on race. Black and Puerto Rican troops were specifically exposed to see how their skin would react. "They said we were being tested to see what effect these gases would have on black skins," Edwards tells Dickerson. Japanese Americans were also tested, as proxies for Japanese troops.

Though the tests themselves are shocking and outrageous, the follow-up to the experiments—or lack thereof—was what ultimately provoked lawmakers to demand restitution for the veterans and their families. The VA has recognized that injured veterans deserve benefits, and NPR's investigation aims to find more eligible victims with its database, which lists the names, last known residences, birth dates, enlistments and military branches where the veterans served.

Those veterans suffered from skin problems, respiratory issues and cancer for decades—and now, some don't trust the VA. When Dickerson interviewed Harry Bollinger, a Navy recruit who participated in the mustard gas tests, he explained how the VA refused to acknowledge his participation in the experiments, citing regulations and a lack of records. After years of rejection letters, when the agency finally recognized that he was exposed to mustard gas, he no longer wanted to go back for his benefits. "I was disgusted already," Bollinger tells Dickerson. "What's the use?"