Deborah Lee James, the secretary of the Air Force, said on Facebook that she was “saddened” by the attack.

“The circumstances for today’s incident are under investigation by the local law enforcement,” she wrote. “Meanwhile, please keep him and his family in your thoughts and prayers.”

Airman Stone and two friends were on a high-speed train bound for Paris in August when, after a stop in Brussels, they heard what sounded like gunfire. After seeing the gunman struggling with another passenger, Airman Stone and two companions, Alek Skarlatos and Anthony Sadler, rushed to subdue him. No one was critically hurt or killed in the attack, and the three were said by the French authorities to have saved many lives.