Washington — The Pentagon is reporting another jump in the number of American troops diagnosed with traumatic brain injuries from the Iranian missile attack in Iraq last month. There are now more than 100 diagnosed and 21 have been sent back to the U.S. for treatment.

CBS News spoke to Sergeant First Class Daine Kvasager days after the attack. His description of what happened helps explain why the number of soldiers suffering concussion-like symptoms continues to climb. He was in the back of a truck operating a drone at the time.

"The truck I was in actually lifted up on two wheels and then came back down," Kvasager said.

President Trump said the symptoms of what is officially called mild traumatic brain injury, were not serious.

"I heard that they had headaches and a couple of other things, but I would say that, and I can report, it is not very serious," he said.

But Kvasager said it felt like a "shock wave," moving through your body. "After you hear it and then, uh, it's just a sinking feeling of like your existence is coming to an end," Kvasager said.

Of the 109 identified with concussion symptoms, 76 have been returned to duty.