But background checks would not have stopped most school shootings, in which the gunmen took legally obtained weapons from parents with spotless backgrounds, says Jack Levin, a professor of sociology and criminology at Northeastern University in Boston.

Many gunmen have a clean criminal history – they are just having a really bad day, he says.

He says most gun deaths are classified as manslaughter, which implies a crime committed without forethought.

“In those cases, the criminal may not have a psychiatric record or a criminal history,” says Mr Levin.

“His pathology is situational. He loses his cool, loses his temper, and in the heat of the moment he takes out his knife from the kitchen drawer or gun from his trench coat and shoots.”