As many as 29 people have been killed and dozens more injured in mass shootings in Texas and Ohio.
Teachers had been warned about Nikolas Cruz, the teenager who is accused of killing 17 people and injuring dozens more at his former school in Florida.
At least 58 people have been killed and more than 515 have been injured in a mass shooting in Las Vegas.
Ten people have died after a gunman opened fire outside a shopping centre in the German city of Munich.
The US Senate has rejected plans to limit the sale of firearms, proposed in the wake of the mass shooting in Orlando earlier this month, which left 49 innocent people dead.