Jack Teixeira followed his family into the military.

While talking with friends online, Jack Teixeira, the air national guardsman who was arrested by federal investigators on Thursday for his role in the leak of a cache of classified U.S. intelligence documents, assumed the role of a leader.

He guided a group of 20 to 30 people, mostly young men and teenagers, as they bonded over guns, racist memes, video games and international politics.

Offline, Airman Teixeira, a 21-year-old from North Dighton, Mass., followed several other relatives into the military after graduating from high school. He even missed his graduation ceremony because of basic training obligations at Lackland Air Force Base in Texas, according to posts on social media.

He finished his technical training the following year and officially entered active duty with the Massachusetts Air National Guard’s 102nd Intelligence Wing in October 2021, according to military service records. He achieved the rank of Airman First Class and was decorated with an Air Force Achievement Medal, which is awarded to junior airmen for notable achievements.

His parents split up when he was young, and he appeared to be close to his sister, who would post baby pictures of Airman Teixeira on social media when she wished him a happy birthday each December. He rooted for the N.B.A.’s Boston Celtics, enjoyed riding all-terrain vehicles and loved his family’s two dogs.

His friends in the online group, a Discord channel named Thug Shaker Central, said in interviews that it was a place that brought together lonely people during the pandemic. Sometimes, when they played war-themed video games, Airman Teixeira wanted to teach his friends about actual war, they said.