From CNN:

Mass shootings that involve multiple shooters are rare in American history: just two of the 28 deadliest shootings were carried out by more than one person. The attack at the Inland Regional Center in San Bernardino, California, is now the third — carried out by a man and a woman whose motives are still unclear. Here is what we know about the attackers: Syed Rizwan Farook and Tashfeen Malik.