Two campus police officers were shot and killed at a college in the US state of Virginia on Tuesday, while one student was killed and another wounded in a shooting at a school in Minnesota the same day. President Joe Biden, who has called on Congress to enact tougher gun control laws, tweeted about the shooting, saying he and his wife “[were] praying for the families of those lost,” and that “gun violence against law enforcement officers is sickening, and it must end”.