An attempted school shooting in the United States has been stopped by an armed policeman. A student opened fire at the school in the state of Maryland before being shot dead. Two students were wounded. This is the 21st incident involving gun violence or a threat of violence in US schools since February 14, when Nikolas Cruz walked into a Florida high school and killed 17 students and teachers with a semi-automatic rifle.
Al Jazeera's Rosiland Jordan reports from Washington.