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.”

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