The grave site of Thomas J. Sullivan, 30, at Arlington National Cemetery. He died in 2009 of pneumonia after years of physical and mental suffering that relatives believe were somehow related to his deployment to Iraq. (Ricky Carioti/The Washington Post)

Dan Sullivan could hear his brother’s dogs howling inside the house when he arrived. His brother, a retired Marine sergeant, had been very sick lately and had stopped answering his phone. So when Dan went to his Falls Church home to check on him and heard the dogs, he got a bad feeling.

He unlocked the door, walked in and found his brother, Tom, dead. He was sitting in his easy chair, a cup of tea, his inhaler and his medical records by his side. His cellphone had fallen under the chair. It looked like he had been gone for a while.

Dan at first thought his brother had killed himself. He had been increasingly sick since his service in Iraq four years before. His ailments seemed endless, and it was never clear exactly what was wrong. People said it must be in his head.

But Thomas J. Sullivan hadn’t committed suicide. He had died at 30 of pneumonia after years of physical and mental suffering that relatives believe were somehow, in some unexplained way, related to his deployment to Iraq.

He had not been wounded or traumatized or wearied by multiple deployments like so many other veterans of the wars in Iraq and Afghanistan.

His case fell into the gray world of the unexplained.

Jeanne Sullivan flips through an album containing photos of her son Tom. He was a single-minded young man who had a great singing voice and was bored by college. He enlisted in the Marines in 2000 and deployed to Iraq in 2004. She says he was in “perfect” health when he left but came back “sick.” (Ricky Carioti/The Washington Post)

Now, five years after his death on Feb. 16, 2009, his family has established the nonprofit, Washington-based Sergeant Sullivan Center for the study of what they term “undiagnosed post-deployment illnesses.”

The maladies, reminiscent of the Gulf War syndrome that emerged after the 1991 Persian Gulf War, are marked by, among other things, an array of respiratory, heart and bowel symptoms, along with chronic pain.

Most seem to defy diagnoses. Tom Sullivan also had high blood pressure, heart disease and colitis, and he was taking a host of potent prescription drugs, including narcotics and steroids, relatives said.

The Sullivans believe that thousands of veterans of the recent conflicts in Iraq and Afghanistan may be similarly affected.

“Deployment-related illness, as something physiological, is not something that a lot of people are aware of,” Dan Sullivan said.

“What we’re really concerned with . . . is not so much what he died of. But how he was treated when he was alive,” Dan Sullivan says of his brother. (Ricky Carioti/The Washington Post)

In 2010, Dan Sullivan, and his and Tom’s parents, Jeanne and Peter Sullivan, founded the nonprofit Washington-based Sergeant Sullivan Center for the study of “undiagnosed post-deployment illnesses.” (Ricky Carioti/The Washington Post)

Tom had been fighting his illness for four years, said his mother, Jeanne Sullivan. “I saw him suffer in terrible pain, terrible fear, because no one was explaining what was happening to him,” she said. “He thought he was alone.”

The family believes that Tom’s sickness and death were direct results of his service in Iraq. They created the center to be a civilian advocate for ailing veterans and service members like Tom, “the private nudge” outside the government box, Jeanne Sullivan said.

“What we’re really concerned with . . . is not so much what he died of,” Dan Sullivan said. “But how he was treated when he was alive.”

‘It was more than that’

After the 1991 Gulf War, thousands of returning veterans began suffering from an array of baffling symptoms.

They included fatigue, joint and muscle pain, gastrointestinal and respiratory problems, memory difficulties, depression and anxiety.

A report last year by the Institute of Medicine, the health arm of the National Academy of Sciences, found that one-third of ’91 Gulf War veterans — an estimated 175,000 to 250,000 people — said they experienced such symptoms.

No exact cause was apparent. Dust? Fumes? Poisons? Pesticides? Medications?

The phenomenon became known as Gulf War syndrome. Doctors preferred the term “chronic multisymptom illness” or “Gulf War illness,” arguing that there was no real syndrome, rather a spectrum of symptoms.

The government spent millions studying and treating those affected, and research and treatment continue.

The Sullivan Center has been raising seed money — $250,000 so far — for research into possible causes and treatments of such undiagnosed illnesses. In April, the center announced a $10,000 matching challenge grant to Georgetown University Medical Center for continued research into a possible Gulf War illness biomarker that could be found in a blood test.

“At this point in time, my aim is to help people,” said James N. Baraniuk, the professor in the Department of Medicine who is to receive the grant. “If we can identify abnormal physiology, identify biomarkers . . . and then find specific treatments . . . then we will have helped them.”

Last year, Pentagon-funded studies at Georgetown suggested that veterans suffering from Gulf War illness have postwar brain abnormalities not seen in those unaffected by the malady, which may cause its symptoms.

“These novel findings . . . provide validation for many veterans who have long said that no one believes them,” researcher Rakib Rayhan said at the time.

So what did Tom Sullivan have?

“At first blush, there seems to be a tie [between Tom Sullivan’s story and] what potentially we saw following the ’91 Gulf War,” Craig Postlewaite, director of force readiness and health assurance at the Defense Health Agency, said in an interview.

Indeed, when the more recent conflicts in Iraq and Afghanistan began, the Pentagon was worried that it would see a repeat, he said.

It didn’t happen, he said.

It’s not clear why. The difference may be that military medical experts now try to treat ailments even if the cause is unclear — something that was not necessarily done two decades ago, Postlewaite said.

“ ‘Let’s not get too hung up on the cause. Let’s get them in and get them treated,’ ” he said the thinking is now. “That, I think, has a lot to do with the fact that we haven’t heard a lot about unexplained illnesses.”

In addition, he said, there is better acceptance of post-traumatic stress disorder, which seems to be “a modulator in the reduction of these unexplained symptoms.”

“We know that the mind controls lots and lots of bodily processes,” he said. “From anxiety and depression, of course, to gastrointestinal symptoms to dermatologic conditions, respiratory conditions, cardiac.”

But with Tom Sullivan, whose story Postlewaite said he knows, “it was more than that,” he said.

“It resulted in his demise,” he said. “Unfortunately, what caused his condition, and others that are out there, we don’t know.”

“It certainly seems to me that . . . what he had very plausibly could be related to his deployment,” he said. But there’s no way right now to find the cause.

For Tom’s father, Peter Sullivan, there is little doubt that war killed his son. “I can’t tell you the exact scientific process. I’m not sure that scientists will ever really do that.”

‘They tried everything’

Tom Sullivan was born in the District and grew up in Silver Spring, the second of the three children of Jeanne and Peter Sullivan, both lawyers, who are now 66 and 68 respectively.

He was a quiet, single-minded young man with a superb singing voice but was bored by college.

He joined the Marines in 2000 and deployed to Iraq in August 2004, serving in Anbar province for several months during a period of heavy fighting, his family members said.

He had a tattoo of a leprechaun on his right forearm and the name of an old girlfriend on his back. He ran and lifted weights. His buddies called him “Sully.”

“He was in perfect health,” his mother said during an interview at the center’s headquarters on Connecticut Avenue in Northwest Washington.

But when he came back, “he came back sick,” she said.

Peter and Jeanne Sullivan, with a photo of son Tom, at their home in the District. The Sergeant Sullivan Center is raising seed money for research into possible causes and treatments of undiagnosed illnesses such as Gulf War syndrome and those that afflicted Tom. (Ricky Carioti/The Washington Post)

“From that point forward, his health deteriorated appallingly,” she said. “His physicians, because they couldn’t explain it, thought it was psychosomatic.”

While he was in Iraq, he had confided to his future wife, Katie, that he had begun suffering from rectal bleeding and abdominal pain, his family said. He put it down to hemorrhoids. But it continued after he came back.

On the post-deployment health-assessment form he filled out on Feb. 17, 2005, he reported that his health had gotten worse.

He wrote that he had been exposed to sand, dust, air pollution, vehicle fumes and smoke from burning trash or feces. He said he had indigestion, chronic congestion and anal bleeding.

Tom and Katie were married in August 2005. They had a daughter, Ava Jeanne, in 2006. Tom was assigned to Marine Corps headquarters in Northern Virginia that December. They rented a house in Falls Church.

But his decline continued.

“I watched Tom lose his health, his career and, worst of all, his dignity as he fought to live with a progressively debilitating illness that the military medical system refused to take seriously,” his wife said in a speech two years ago.

He eventually had a colonoscopy to check his bowel problems, but it had to be stopped because his intestines were found to be ulcerated, family members said.

Symptoms came and went, puzzling doctors, relatives said. Other problems cropped up. He suffered from asthma, insomnia, joint pain and sleep apnea. He was treated for Crohn’s disease, a severe bowel inflammation, and fibromyalgia, a stress-related pain and sleep disorder.

“They tried everything,” his father said. (Dan said Tom was never under the care of the Department of Veterans Affairs, which has been severely criticized for its handling of veterans’ health.)

The month before he died, he had more than 20 drug prescriptions. His wife, who was not available for an interview for this article, said in her speech that he was taking 15 oxycodone pills a day.

“At one point, his doctor told me that he was on as much pain medication as a terminal cancer patient,” she said. “He was in pain all the time and never wanted to be touched.”

Tom was sent to a clinic at what was then Walter Reed Army Medical Center that treated people with psychosomatic illness, Dan Sullivan said. It was set up to try to deal with Gulf War syndrome.

“The problem is . . . the patient [then] starts to think, and the doctor starts to think, that it’s all in the head of the patient,” he said.

“When all these folks started coming back from the Persian Gulf War with weird symptoms, a kind of narrative was created to explain these symptoms as being psychosomatic,” he said. “And when my brother came back and a lot of these other folks came back, they were put into that story.

“That narrative is damaging. Military service [is] an occupation with lots of environmental, toxic hazards, not just bullets.”

Tom was medically retired in 2008 but continued working for the Marines as a civilian up to the week before he died.

“His final year was torture,” his mother said. He had put on almost 100 pounds, and his blond hair had turned gray. At one point, “he was actually on the floor, crying from the pain” in his hips, back and knees, she said.

“He was afraid he was dying, and his doctors didn’t think he was even very sick,” she said.

But Dan Sullivan said the family, at the time, was largely unaware of how ill Tom was. “His doctors had effectively persuaded all of us, including him, that these symptoms were a nuisance,” he said.

“So, frankly, you start to get mad at him for being sick,” he said. “It was just interminable, the whole thing.”

A few weeks before Tom died, Dan got so frustrated that he urged his brother to see a psychiatrist: “He didn’t respond when I said that. And I now understand why that might have made him angry, because that’s all he had been getting from everyone.”

The day he died, his wife and daughter were attending a funeral in Chicago, and his parents were out of town. So Dan went to check on him.

When he found Tom, he called 911 and was told that if Tom was not breathing, he should leave the house. Dan hesitated. “I didn’t want to leave him alone in the room,” Dan said. He stayed until paramedics arrived and confirmed that his brother was dead.

Five days later, a memorial service was held in Georgetown. There was a big crowd, and everyone sang “Amazing Grace,” one of the hymns he had sung so well.

On March 4, he was buried in Arlington National Cemetery in Section 64, adjacent to Section 60, where many of his comrades from the wars in Iraq and Afghanistan are also laid to rest.