Armed with assault-style weapons, the two are believed to have opened fire yesterday morning at a social services agency holiday event in San Bernardino, California. The attack, which also left 17 wounded, was the most deadly shooting in the United States in three years.

Farook was a employee of San Bernardino County who attended a holiday party at the Inland Regional Center, stormed out and later returned to fire on the celebration, said Jarrod Burguan, police chief in the city of San Bernardino, 60 miles (100 km) east of Los Angeles.

Yesterday's carnage differed from most other recent U.S. killing sprees in key ways, including the involvement of two people rather than a lone perpetrator.

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.

Read more: 14 killed in California shooting; one suspect identified

In pics: California shooting rampage

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