(CNN) -- Some of the children killed at Plaza Towers Elementary School in Moore, Oklahoma, during Monday's storm drowned in a basement area there, Oklahoma Lt. Gov. Todd Lamb told CNN Tuesday morning. "My understanding, this school ... Plaza Towers, they had a basement. Quite frankly, don't mean to be graphic, but that's why some of the children drowned, because they were in the basement area," he said. Officials have said the storm killed at least seven children at the school.