Mass shootings such as last week’s in Florida have prompted law enforcement and the media to look for patterns, and here’s one that has emerged: A disturbing number of perpetrators have previously been accused of domestic abuse. Everytown for Gun Safety, a gun-control-advocacy group, analyzed FBI data on mass shootings from 2009 to 2016 and found that in 54 percent of cases, the victims included the shooter’s current or former spouse or intimate partner, or another family member — and 16 percent of attackers between 2009 and 2015 had previously been charged with domestic violence.