A spate of shootings over the past several days has law enforcement on edge, with some warning that a turbulent brew of a pandemic, racial unrest, historic surges in gun sales and a rancorous election year could make it an especially deadly summer. Although mass shootings are down sharply this year, other non-suicidal gun deaths are on pace to exceed last year, according to incidents tracked by the Gun Violence Archive. “There’s something going on at the moment, these underlying tensions,” said James Densley, professor of law enforcement and criminal justice at Metropolitan State University.