Caitlin Dickerson

The killing of Alton Sterling, 37, by police earlier this week touched off protests across the country – but in Sterling's home city of Baton Rouge, La., demonstrators' outrage has rarely exceeded a parboil. And that's by design.

In its broad outlines, we know this story: mass shooting, dozens dead, more injured.

But authorities are still trying to piece together exactly what happened at Pulse nightclub in Orlando, Fla., nearly a week ago, in what is the deadliest mass shooting in modern U.S. history.

The chaos unfolded quickly inside the nightclub late Saturday night, where Omar Mateen gunned down 49 people and injured more than 50 others.

Sen. Claire McCaskill, D-Mo., plans to introduce legislation today to help World War II veterans who were exposed to mustard gas. The vets were used in classified experiments conducted by the U.S. military, and were sworn to secrecy about their participation for a half-century.

Charles "Lindy" Cavell could never forget what the U.S. military tried to hide. Cavell fought to bring to light the secret mustard gas testing program he had participated in during World War II and for VA compensation for the test subjects. He died at home Wednesday at 89.

In response to NPR's investigation into once-classified U.S. military experiments with mustard gas, we've been hearing from people who learned about their loved ones' experiences in the testing.

To understand the predicament of World War II veterans exposed to mustard gas, take a look at what happened to another set of American veterans who were exposed to a different toxic chemical.

Last month, NPR reported that some of those World War II vets are still fighting for disability benefits from the U.S. Department of Veterans Affairs because the agency says they don't have enough proof to substantiate their claims.

This week, NPR reported that the Department of Veterans Affairs failed to live up to a promise to contact 4,000 veterans who were exposed to mustard gas in secret military experiments. In 1993, the VA promised it would reach out to each of those veterans to let them know that they were eligible for disability benefits. Instead, over the past 20 years, the VA reached out to only 610.

In secret chemical weapons experiments conducted during World War II, the U.S. military exposed thousands of American troops to mustard gas.

When those experiments were formally declassified in the 1990s, the Department of Veterans Affairs made two promises: to locate about 4,000 men who were used in the most extreme tests, and to compensate those who had permanent injuries.

But the VA didn't uphold those promises, an NPR investigation has found.

As a young U.S. Army soldier during World War II, Rollins Edwards knew better than to refuse an assignment.

When officers led him and a dozen others into a wooden gas chamber and locked the door, he didn't complain. None of them did. Then, a mixture of mustard gas and a similar agent called lewisite was piped inside.