Authorities were trying to determine Friday how an 8-year-old boy obtained a loaded handgun that was found in his backpack by school police after he made threats toward a classmate.

The third-grader at Sharp-Leadenhall Elementary School, a small Baltimore City school for special-needs children, was arrested Thursday afternoon and charged as a juvenile with handgun possession. School officials said the boy was “acting suspiciously” and staff began closely monitoring his behavior, which led to a search of his backpack and the discovery of a .380-caliber handgun.