Sgt. Klayton Thomas looked every bit the poster boy Marine as he strode into a military hospital last September to get his back checked.

He taught karate and earned his abs in the gym. He had survived a 2007 deployment to Iraq, even thrived during his prolonged stay in the middle of the then-treacherous Sunni Triangle. He rarely drank. He didn't smoke. Life seemed perfect on this mid-September Thursday, if only his back would stop aching. The 25-year-old Columbus, Neb., native thought he had wrenched it playing soccer. Three months and 10 days later, he died in hospice care.

This much is known: Thomas succumbed to an unstoppable lung cancer that crushed his vertebrae, blitzed his bones and invaded his brain, dumbfounding doctors who had spent their entire careers treating the disease.

His death leaves a medical mystery, one similar to those posed by hundreds of other American military personnel battling exotic cancers or struggling with rare respiratory problems.

This mystery begins in the unlikeliest of places: Iraqi “burn pits” — large, primitive landfills where contractors set trash aflame, causing ever-present black smoke to drift over dozens of U.S. military bases.

Health experts, a high-powered defense lawyer, Congress and even the president have taken notice, asking questions like Klayton Thomas' parents and doctors asked in the weeks after he fell ill.

Why would an otherwise healthy young nonsmoker contract a cancer that generally haunts older smokers? Why did this cancer spread like wildfire when experts say its normal path can take years?

Simply put: Why did Sgt. Klayton Thomas die?

“We were scared to death when he went to Iraq, scared of a mortar attack, an IED,” said his mother, Connie Thomas of Columbus. “But nothing like this. Not in our wildest dreams.”

* * *

Just before Halloween, Thomas and his parents met Dr. Ray Lin at San Diego's Scripps Medical Center. A month had passed since doctors first found white spots on Thomas' lungs, and as the Marine and his parents took their seats in the radiologist's office, they felt as if they now lived inside a never-ending nightmare.

First the cancer had spread into Thomas' spine, his hips, his shoulder blades. Then he had endured his first chemo treatment and an excruciating back surgery to put cement into his crushed sixth vertebra.

The pain had gotten so severe he couldn't sleep, even with the aid of morphine, and could barely move without a walker.

Thomas' wife could comfort him only by cell phone — Mia, a Filipina whom Thomas had married while stationed in the Philippines, was struggling to secure her American visa.

But the blackest day had come on the last day in September, on Thomas' first visit to the highly regarded Scripps hospital.

On that day, Dr. Robert Sarnoff, president of the facility's medical group, had delivered the news:

Sgt. Thomas, he said, this is bad. You have a 5 percent chance to live.

Connie and Dave Thomas had flown from Columbus to San Diego to fight through layers of military bureaucracy and secure their son special treatment at the nonmilitary hospital.

Now Lin pulled out an X-ray of Thomas' shoulder. A healthy shoulder X-ray should show up white. The X-ray Lin held was shrouded in black.

Lin told the family that Thomas could have a genetic predisposition to cancer — his father and several uncles had survived various cancers in middle age.

But there must have been an additional trigger for the cancer to spread this quickly, he said, according to the Thomas family. (Lin was out of the country, according to a hospital spokesman, and privacy laws prevent Scripps from discussing Thomas' case.)

Have you been exposed to something toxic, Klayton?

Thomas sat silently for a moment and then told the doctor and his shocked parents about the burn pit near where he lived and worked at al-Taqaddum Air Base in Iraq.

He told them about the hazardous materials burned there. He told them the smoke sometimes darkened the sky and grew so thick it choked him.

That night, his mother, a nursing home administrator, sent out one of her mass e-mails updating family and friends on Thomas' condition. Usually she wrote these e-mails resolutely, marking them with hope that a miracle could occur, that her son would recover.

Not that night.

“I can't understand why God is allowing this to happen,” she wrote.

* * *

On Nov. 6, the burn pit movement got its day on Capitol Hill.

The chief allergist of a New York veterans hospital testified that Americans who deploy to Iraq were twice as likely as other veterans to develop respiratory illnesses, according to his four-year study.

Others who testified cited a large group of Kentucky soldiers found to suffer from bronchiolitis, which can irreparably damage the lungs, after exposure to a particularly toxic Iraqi fire in 2003.

They ridiculed the military's previous burn pit studies. One tested for air quality during Iraq's wet season, which, according to the VA hospital's allergist, “is like testing for snow in Albany during the summer.”

In response, Rep. Tim Bishop, D-N.Y., introduced a bill that would create a complete list of burn pits, mandate a registry for all troops exposed to the pits and give those troops special physical exams. It is designed to build on a previous Bishop proposal, passed into law, that seemingly barred the use of most burn pits in Iraq, though dozens are still operating.

“There is just too much evidence that these burn pits are hazardous for (the military) to continue to ignore it,” Bishop told The World-Herald.

It's also getting harder to ignore a class-action lawsuit originally filed in Texas in December 2008.

Since then, more than 300 service members and contractors in 42 states have joined the multimillion-dollar lawsuit, which alleges that burn pits run by the military contractor KBR (formerly Kellogg Brown and Root) caused their health problems.

Most who have joined the lawsuit are suffering from pulmonary illnesses, said Susan Burke, the group's lawyer.

A smaller number of military personnel joined the suit after being diagnosed with cancer, which they say developed because of known carcinogens burned in the pits.

Ten of those with cancer have died. Burke said most of them were in prime physical condition but succumbed in months.

“These are young men and women who voluntarily went off to fight for all of us, and an American company poisoned them,” Burke said.

KBR lawyers say the company followed military protocol when it designed and operated the burn pits. There's no definitive proof that a burn pit directly harmed anyone's health or caused the myriad symptoms described in the lawsuit, they contend.

The sick service members and contractors sense that no matter the outcome of the lawsuit, they are turning a corner with the U.S. government.

VA Secretary Eric Shinseki told military reporters last year that his administration would not repeat the errors of Agent Orange. For decades, the military denied that the herbicide — used to destroy dense jungles during the Vietnam War — caused sickness. Eventually officials admitted the link between Agent Orange and the illnesses of thousands of veterans.

President Barack Obama said last year that his administration had no interest in “sweeping things under the rug.”

And, on Dec. 16, R. Craig Postlewaite, the American military's senior health protection official, publicly acknowledged that the burn pits had probably caused serious illness.

“We feel at this point in time that it's quite plausible — in fact, likely — that there are a small number of people that have been affected with longer-term health problems,” he told the Salt Lake Tribune.

The Department of Defense is launching a more comprehensive study that could further validate outside research that indicates the burn pits have sickened troops. And the military has installed incinerators at Balad, closing Iraq's most infamous burn pit.

“At times you feel like you are battling this all alone,” Elizabeth Hilpert said. “But the story is changing.”

* * *

Sgt. Klayton Thomas won't see it end.

Two days after Christmas — two weeks after the military acknowledged a probable link between the burn pits and serious illness — Thomas walked from his bathroom to the living room recliner where he spent all his time.

He sat back down, looked toward his reading lamp, gasped twice and stopped breathing.

His family buried him on a frigid Saturday in January.

Bundled-up Marines carried the flag-draped casket. They fired a 21-gun salute into the air. They hugged his mother and his widow.