(Reuters) - In the wee hours of Jan. 8, Tehran retaliated over the U.S. killing of Iran’s most powerful general by bombarding the al-Asad air base in Iraq.

FILE PHOTO: U.S. soldiers inspect the site where an Iranian missile hit at Ain al-Asad air base in Anbar province, Iraq January 13, 2020. REUTERS/John Davison

Among the 2,000 troops stationed there was U.S. Army Specialist Kimo Keltz, who recalls hearing a missile whistling through the sky as he lay on the deck of a guard tower. The explosion lifted his body - in full armor - an inch or two off the floor.

Keltz says he thought he had escaped with little more than a mild headache. Initial assessments around the base found no serious injuries or deaths from the attack. U.S. President Donald Trump tweeted, “All is well!”

The next day was different.

“My head kinda felt like I got hit with a truck,” Keltz told Reuters in an interview from al-Asad air base in Iraq’s western Anbar desert. “My stomach was grinding.”

Keltz, who said he had concussion symptoms for days, is among 109 soldiers diagnosed with traumatic brain injuries in the wake of last month’s attack, a figure that has steadily risen as more troops report symptoms and get medical screening.

Reuters interviewed more than a dozen officials and soldiers and spoke with brain-injury specialists to assemble the most comprehensive account so far of the nature of the soldiers’ injuries and how they sustained them.

The slowly rising casualty count underscores the difficulty in detecting and treating what has become one of the most common injuries in the U.S. military during two decades of war in Afghanistan and Iraq, where U.S. troops face roadside bombs, rockets and mortars.

More than a week after the attack, on Jan. 16, Defense Secretary Mark Esper was made aware that soldiers had suffered brain injuries from the missiles, the Pentagon said. That day, the Pentagon reported that an unspecified number of troops were treated for concussive symptoms and 11 were flown to Kuwait and Germany for higher-level care.

On Jan. 22, Trump said that he “heard that they had headaches and a couple of other things,” prompting criticism from both Democratic and Republican lawmakers and a U.S. veterans group that the president was underplaying the casualties from the attack.

“I think it was unfortunate to use those words,” said Republican Representative Richard Hudson, who represents Fayetteville, North Carolina, home to Fort Bragg that includes the Army’s Special Operations Command.

The White House declined to comment for this story.

A DIFFERENT CLASS OF WOUNDS

The U.S. military has long treated brain injuries as a different class of wounds that do not require rapid reporting up the chain of command, unlike incidents threatening life, limb or eyesight.

Since 2000, nearly 414,000 service members have been diagnosed with traumatic brain injuries, or TBIs, according to Pentagon data. The number is likely higher because the Pentagon only counts as one injury cases where a soldier suffers brain trauma in multiple incidents.

U.S. troops operating drone flights appeared to have suffered the most brain injuries during the attack on al-Asad, said Lieutenant Colonel Timothy Garland, who was on the base at the time. Many worked near the air strip, he said. Like Specialist Keltz, who was manning a guard tower, the drone pilots had been assigned to watch for a possible ground attack.

“Those drone pilots, they’re the ones that took the brunt of the TBI cases,” said Garland, who as commander of Task Force Jazeera oversees more than 400 soldiers.

The number of troops diagnosed with brain injury from last month’s attack was expected to stabilize near the current count, one U.S. official said. Less than 10 were now being monitored with possible TBI symptoms, the official said.

The total U.S. military count, however, excludes civilian contractors on the base at the time, many of whom have since departed.

Some U.S. troops also suffered from anxiety-related symptoms after the attack, including sleeplessness and, in at least one case, a sustained high heart rate, according to interviews with soldiers and officials. However, they could not provide a specific number.

The Pentagon categorizes brain injuries as mild, moderate, severe or penetrating. The vast majority of injuries are classified as mild, as were all of the injuries reported from al-Asad.

STANDING GUARD

Garland, the commander, said he was taken aback when he learned of U.S. intelligence indicating that Iranian missiles would strike within hours. He immediately found a base map and started sizing up the best options to shelter his troops.

He recalled old bunkers on the base built during the era of Saddam Hussein, the Iraqi dictator toppled by U.S.-led forces in 2003. But the bunkers wouldn’t hold everyone. Some would need to disperse, taking cover in armored vehicles driven away from targets.

Others in Garland’s unit -- including Specialist Keltz --would need to stand guard to watch for additional attacks beyond the expected missiles.

Keltz said he and a fellow soldier were already manning a tower when First Sergeant Larry Jackson came to them, explaining the intelligence and giving them their orders.

“What I need you boys to do is to lay down on the ground when the impacts happen - and then I need you to jump right back up and man those guns,” Jackson said in an interview, recounting his instructions to Keltz and other soldiers at the base.

As the Iranian missiles streaked through the night sky toward the base, their engines glowed orange - like the ends of lit cigarettes, Garland said. The glow was all that Garland could see in the darkness before scrambling back into a bunker.

Then came the blasts. At least eleven missiles struck the base, destroying housing units made from shipping containers and other facilities.

“Every explosion I heard, I was thinking, OK, that’s a number of people that have just lost their lives,” he said.

But initial checks after the attack showed nobody was killed or obviously injured, despite massive devastation to the base. Word got back to Washington. Just before 6 a.m. in Baghdad, Trump tweeted an update: “Assessment of casualties & damages taking place now. So far, so good!”

FALLING THROUGH THE CRACKS

On the ground at al-Asad, U.S. Army Major Robert Hales, a doctor who is deployed to al-Asad, defended the initial reports of no injuries.

“Everyone here did not have any outward physical injuries,” he said in an interview. “There were no lacerations. There’s no shrapnel wounds.”

Such “silent” injuries take time to manifest, he said.

Injury figures kept climbing in the weeks after the attack. What began as at least 11 cases grew to 34 about a week later.

On Jan. 22, Trump made his controversial comment, referring to the injuries as “headaches.” The Veterans of Foreign Wars demanded an apology for Trump’s “misguided remarks”.

A week later, on Jan. 28, the toll of brain injuries climbed to 50. In early February, Reuters was the first to report that the count had surpassed 100.

The brain injuries sustained in the Iranian missile attack are fundamentally different than those that have typically resulted from past attacks, brain-trauma specialists said.

That’s because the al-Asad bombing was more intense than typical quick-hit, single-explosion attacks: The explosions came in waves and lasted more than an hour.

When a roadside bomb goes off in Afghanistan, head wounds are often visible. In insurgent bomb blasts, shrapnel or other flying debris can cause brain injuries upon impact. But the damage from large pressure waves from a major blast - like the ones at al-Asad that Specialist Keltz felt - often take more time to diagnose.

Marilyn Kraus, director of the Traumatic Brain Injury program and concussion clinic at George Washington University, said troops may minimize or underreport their symptoms initially. Others may not show symptoms until much later in part because their injuries are initially masked by the adrenaline rush that comes with combat.

“Some of these things can fall into the cracks initially,” said Kraus, who previously served as medical director of the Traumatic Brain Injury Consult Section at the Walter Reed military hospital in Bethesda, Maryland.

In the short term, mild traumatic brain injury can cause headaches, nausea, dizziness and confusion, while longer-term effects can include chronic headaches, mood changes and dizziness, Kraus said. Repeated head injuries can lead to Chronic Traumatic Encephalopathy, a brain degeneration disorder that some researchers have linked to suicidal thoughts, substance misuse and depression, she said.

Hales, the Army doctor, cited research within the past six months showing in animal models that signs of damage to the brain can increase in the weeks after a blast. At al-Asad, soldiers started showing symptoms such as headaches or a “foggy feeling” days after the attack, Hales said. The symptoms often persisted.

“That’s the reason why you saw a huge delay” in identifying the injuries, he said. “That prompted us to re-screen pretty much the whole population of al-Asad.”