The motives of U.S. mass killers, by the numbers

STORY: The attackers were overwhelmingly men, often with histories of mental health symptoms, financial insecurity or engaging in domestic violence. Guns were typically the weapon of choice.

The report comes days after a pair of mass shootings in California took the lives of 18 people and as authorities searched for motives in the attacks, both linked to older men.

The 70-page report, issued on Wednesday by the Secret Service's National Threat Assessment Center, examined 173 incidents where three or more people were harmed. The attacks targeted workplaces, schools, religious institutions and public transportation, among other locations, killing 513 people and injuring 1,234.