Nikolas Cruz's biological mother was a drug addict who gave birth to his brother in prison, according to a published report.
When she learned his biological mother was pregnant again with another boy, she reportedly adopted him for a $15,000 fee. The child, Zachary, was born in prison, according to the report.
Lynda Cruz and her husband, Roger, had struggled to have children of their own. Roger died of a heart condition in 2004, when Nikolas was just 5. Lynda died of pneumonia last November.
Nikolas and Zachary, now 18, went to live with family friends.
Nikolas Cruz, 19, is accused of entering Marjory Stoneman Douglas High School in Parkland, Fla., on Feb. 14 and killing 17 people.
Cruz, who is said to have etched Nazi swastikas on his ammo clips, allegedly planned to set up a "sniper's nest" to shoot at students as they fled from the school, according to CBS News. He allegedly tried to smash a window by firing bullets at it, but the hurricane-proof glass would not break.
He allegedly stopped firing only when his weapon jammed.
Cruz had reportedly taken an Uber to the school. The driver, who asked not to be identified, told CBS News that she recalled him carrying a guitar case and was unaware there was an assault rifle inside.
"I saw him with a backpack that I think it was a guitar case," she said. "And he told me, 'I am going to my music class.'"
Cruz is charged with 17 counts of premeditated murder.