He was a baby on 9/11. Now he’s one of the last casualties of America’s longest war.

After Lance Cpl. Rylee McCollum, 20, landed in Afghanistan with his Marine unit, his father, Jim, began checking his phone for a little green dot. Mr. McCollum had not been able to talk with his son, but the green dot next to Rylee’s name on a messaging app meant that he was online. That he was still OK.

When news came that a suicide bomber killed 13 American service members outside the airport in Kabul on Thursday, Mr. McCollum checked again for the dot. His son was on his first overseas deployment, had gotten married recently, and was about to become a father. Mr. McCollum messaged his son: “Hey man, you good?”

But the green dot was gone.

“In my heart yesterday afternoon, I knew,” Mr. McCollum said.

On Friday, Lance Corporal McCollum became one of the first American victims to be publicly identified in the attack that also killed at least 170 Afghans. It was the highest U.S. death toll in a single incident in Afghanistan in 10 years. His death was confirmed by his father and by the governor of Wyoming, Mark Gordon.

While the Department of Defense has not released an official accounting of the victims, their names began to emerge on Friday. They appeared in social media posts from family and friends and somber announcements from the high schools where the young men had played football or wrestled just a few years earlier.

Some of them, like Lance Corporal McCollum, who was born in February 2001, were still babies when the United States invaded Afghanistan. Others were not yet born. Now, they are among the last casualties of America’s longest war.