(CNN) The younger brother of the man who killed 17 people at Marjory Stoneman Douglas High school told investigators that he bullied his sibling when they were younger, according to a sheriff's report.

Zachary Cruz said two days after the massacre that he felt guilty about the incident and thought he possibly could have prevented it, a Palm Beach County detective wrote in the report.

Cruz, who has turned 18 since the shooting, told the investigators from Palm Beach and Broward counties that he and his friends had bullied Nikolas Cruz, something Zachary "now regrets ever doing," according to the report.

Zachary wishes that he had been nicer to his brother, the detective wrote. Zachary Cruz also may have had some resentment because Nikolas Cruz may have been the "favored brother," the report said.

Nikolas Cruz used a semiautomatic rifle to kill 14 students and three teachers on February 14 at Stoneman Douglas in Parkland, Florida. The shooting spurred a national debate over gun laws.