“Usually with things like this, you get a letter or two from a mother or father, but this is a step-by-step rundown of everything that happened,” Mr. Hughes said. “It’s like reading a novel.”

According to the memo, Mr. Saidakhmetov was born along the old Silk Road in the “backwater city” of Turkestan, Kazakhstan. His father, Arbor Yuylanov, was an alcoholic who beat and raped his mother, Saeira Gulyamova, the memo said. When Mr. Saidakhmetov was 4, his mother fled the marriage and took him to Chernyak, her hometown farming village on the edge of the Kazakh steppe. After that, he rarely saw his father, usually, the memo says, roaming drunk on Turkestan’s streets.

Though Chernyak had a mosque, religion played a limited role in Mr. Saidakhmetov’s childhood. His family identified as Muslim, but did not attend the traditional Friday prayers. His life became even more secular when, after months in Chernyak, his mother moved him to Tashkent, the capital of Uzbekistan. There, she worked long hours as a journalist and left him in their apartment. Alone most nights until 9, the memo said, Mr. Saidakhmetov learned to light a stove when he was 7 and often ate solitary dinners, waiting for his mother.

Image His lawyers said, that “young and lost in modern America,” Akhror Saidakhmetov took to the internet, immersing himself in religious videos and ultimately embracing the Islamic State.

The family might have stayed in Tashkent. But Ms. Gulyamova’s journalism work eventually ran her afoul of the Uzbek government. She moved to Brooklyn in 2010, settling in the neighborhood of Brighton Beach and taking a job cleaning houses. Mr. Saidakhmetov remained behind in the care of an aunt. But within about a year, he joined his mother in New York.

He was 16 at the time, living in an unfamiliar country where he barely spoke the language and had no friends or family beyond his mother. He enrolled at James Madison High School, but for his first six months, the memo said, did not understand a word that was said.

In his second year at Madison, prosecutors say, Mr. Saidakhmetov attacked a safety officer and was expelled. A few months later, he transferred to Abraham Lincoln High School, but did not last long there either. “The New York City public schools didn’t have the resources or energy to help him,” said his lawyer, Adam Perlmutter. “So he dropped out.”