Zachary Cruz, the younger brother of Florida shooter Nikolas Cruz, says he regrets bullying his older brother and feels guilty he didn't stop the massacre of 17 people.
CNN reported the 18-year-old said he and his friends had bullied Nikolas, something Zachary "now regrets ever doing".
One detective wrote Zachary wished he had been nicer to his brother. Police said Zachary had felt resentment because he thought Nikolas was the "favoured brother", reports news.com.au.
The police report also stated Zachary felt guilty about the shooting and thought he could have prevented it.
The two brothers were adopted by Lynda and Roger Cruz. But the father died of a heart attack several years ago, and Lynda passed away from pneumonia in November.
Nikolas Cruz used a semiautomatic rifle to kill 14 students and three teachers on February 14 at Stoneman Douglas in Parkland, Florida.
Nikolas made a 911 call in the days following his mother's death and told police he was struggling, according to the Miami Herald.
"I lost my mother a couple days ago and I'm dealing with some things right now. I kind of got mad and started punching walls."
The police were called to his home nearly two dozen times in the decade leading up to deadly attack, and two people called the FBI about Nikolas but nothing was done.
The Parkland shooting has prompted school students to call for stronger gun control in the US.