Hundreds of FBI officials in dark suits bowed their heads in a Virginia chapel Friday as a pastor offered a prayer to God: “Your son gave his life so that we may live.”

Before him and in the center of the vaulted space was the coffin of David LeValley, draped in an American flag. LeValley, the FBI’s top agent in Atlanta, died in May of cancer brought on by his time as a first responder in the ruins of lower Manhattan after 9/11.

LeValley later told friends he thought he was going to die that morning as the first of the twin towers came down in front of him. But the terror attack would not claim him as a victim until nearly 17 years later — long after the months he spent pulling survivors and evidence from the rubble of “the pile,” after he raised three children and built a career as a thoughtful and selfless leader, his colleagues eulogized.

He was buried in a military ceremony at Quantico National Cemetery. He was 53.

Fifteen current and former FBI officials have died since 9/11 as a result of illnesses caused by the toxins they inhaled in their work at the attack sites.

Earlier in June, former FBI supervisory special agent Brian Crews, also 53, died of cancer, years after serving on an evidence response team at a landfill housing debris from Ground Zero. Crews was the third current or former agent in only the past few months to be recognized with this kind of line-of-duty death.

“They weren’t thinking of the danger, they weren’t thinking about their own safety, nor were they concerned for it at that time. They were concerned for the safety of others,” FBI Deputy Director David Bowdich said Friday at the Virginia memorial service. “We’re only beginning to understand the long-term impact of that work on all our first responders.”

In addition to the 343 New York City firefighters that died in the initial collapse of the towers, another 176 men and women from FDNY have died from 9/11 health complications. The New York City Police Department counts 136 of its own who have died from illnesses related to the attacks.

As the World Trade Center towers collapsed, a dense toxic cloud of dust that included asbestos, cement, glass shards, and over 91,000 liters of jet fuel enveloped lower Manhattan. It is believed that 60,000 to 70,000 first responders breathed those toxins in during rescue and clean-up efforts.

In the immediate aftermath of the attacks, doctors saw irritant-related responses.

“Early on we picked up asthma, sinus (problems), heartburn from breathing and swallowing the stuff,” said Dr. Michael Crane, director of the World Trade Center Health Program Clinical Center of Excellence at Mount Sinai.

According to the Centers for Disease Control and Prevention’s World Trade Center Health Program, sinus issues, reflux and asthma are the three most common 9/11-related health issues among first responders and survivors of the attack.

The program tracks and provides medical monitoring and treatment for emergency responders, workers, volunteers and survivors of the 9/11 attacks. Other 9/11 related illnesses include post-traumatic stress disorder, depression and chronic respiratory issues.

Nearly two decades since the attacks, doctors are now seeing a shift in the types of diseases they are diagnosing.

“The first wave was the acute — the deaths and all the acute injuries in the first couple of days. The second wave was the aftermath and developed the sinus (issues), asthma, anxiety and depression,” said Dr. Jacqueline Moline, director of the Queens World Trade Center Health Program at Northwell Health.

And now, she said, the illnesses have taken on a new dimension. “The third wave is the diseases that take years to manifest. The transformation from acute to chronic to permanent disease. And that’s where we really are now,” she said.

As of April 30 of this year, over 43,000 people have at least one 9/11-related health issue. Over 80% of that group are first responders. In the past few years, the number of cancers has climbed.

“Cancer has a latency period. The earliest cancers are the blood cancers. And then you see solid tumors,” said Moline. “So the fact that we’re seeing tumors on the uptick is not a surprise at all.”

In the beginning of 2015, there were 3,204 cancers in the World Trade Center Health Program’s registry. By the end 2016, that number had almost doubled to 8,188 incidences. World Trade Center Health Program figures shared with CNN now count more than 9,300 cases of cancer as of April 30.

Studies have found that New York firefighters who responded to the 9/11 attacks were 10% more likely to develop cancer than the general population and that they are more likely to develop myeloma precursor disease, which can develop into cancer.

And while it’s hard to tease out just how many of these cases of cancer are directly a result of the attacks, aside from other factors such as age and family history, there is no question that 9/11 hastened some of these incidences. Additional exposure led to “more opportunities for the genetic formations,” Moline said.

“We’re hoping that might be a temporary trend, but it looks like it is continuing to increase,” said Mount Sinai’s Crane. In his clinic, he is now signing off on more than 20 cancer diagnoses related to 9/11 every week, he said.

“It’s definitely concerning to see people pass in a very short period of time,” said Thomas O’Connor, the president of the FBI Agents Association, a group that encompasses more than 80% of active duty agents.

O’Connor responded for months to process the scene at the Pentagon, where a hijacked plane crashed into the building on September 11, 2001, damaging a large area and killing 125 people inside.

“We were all wearing after the first day, or in part of the first day, masks — paper masks,” O’Connor said. “Even with the personal protective equipment, it still didn’t make a difference really.”

He’s healthy now, but goes in each year for “a whole myriad of testing” through the CDC health program. Once a year, an envelope with the results arrives in the mail.

“It’s like a bad lottery day — you hope you don’t win this one,” he said. Some 14 agents are currently sick and enrolled in the program, O’Connor said.

At the FBI, O’Connor, along with his wife and a bureau friend, successfully petitioned then-FBI Director James Comey to formally recognize and memorialize agents who died from 9/11-related causes, he said.

Families of agents who died of 9/11-related causes now qualify for some line-of-duty death compensation, and can earn a spot on the venerable Wall of Honor. The agents association also runs a scholarship fund that has paid out millions in college costs to the children of fallen responders.

Today, the agents association is fighting to win more coverage from federal agencies for agents that are becoming sick. They are also working to remind the public about the work of the FBI and the sacrifice made 17 years ago.

“In this time of FBI bashing, I think it’s important for people to know that the agents continue to do this type of work, they continue to die for the work they’ve done, and they’re going to continue to do that,” O’Connor said.