Spent cartridge lies on the ground as police officers secure the area after at least one person opened fire at a social services agency in San Bernardino, California, Dec 2, 2015.[Photo/Agencies]

STRING OF SHOOTINGS

So far in 2015, there have been more than 350 shootings in which four or more people died, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of mass shootings.

The shooting in California comes less than a week after a gunman killed three people and wounded nine in a shooting rampage at a Planned Parenthood clinic in Colorado Springs, Colorado. In October, a gunman killed nine people at a college in Oregon, and in June, a white gunman killed nine black churchgoers in South Carolina.

Gun control advocates, including Democratic President Barack Obama, say easy access to firearms is a major factor in the shooting epidemic, while the National Rifle Association and other pro-gun advocates say the Second Amendment of the US Constitution guarantees Americans the right to bear arms.