The boy should have been questioned by police and an immediate investigation opened instead of turning the boy over to juvenile authorities, who placed him on community detention, police told The Baltimore Sun in a story published Wednesday.
The delay last week made it difficult to trace ownership of the weapon and put the city at risk, police spokesman Anthony Guglielmi said.
"We would have appreciated a little more notice so we could have gotten a jump-start on the gun aspect of this case," Guglielmi said.
Schools officials at Sharp-Leadenhall Elementary School cited the boy's age and his status as a special-needs student in their decision to proceed cautiously, the Sun reported.
No arrests had been made in the case as of late Tuesday.