According to the FBI, there were two perpetrators. Syed Rizwan Farook who also worked at the Department of Public Health and his wife, Tashfeen Malik. The couple were "homegrown violent extremists" inspired by foreign terrorist groups. They had amassed a large stockpile of weapons, ammunition, and bomb-making equipment in their home.
Farook was a Muslim and an American born citizen of Pakistani descent. Malik was a Muslim and Pakistani-born green card holder.
After the shooting, the couple fled in a rented Ford Expedition SUV. Four hours later, police pursued their vehicle and killed them in a shootout, which also left two officers injured.
Enrique Marquez Jr., a friend and former neighbor of Farook's, was arrested in December 2015, and later pleaded guilty to federal charges of providing material support for terrorism.
Marquez admitted that, in 2011, he conspired with Farook to carry out shooting and bombing attacks. Three other people, including Farook's brother and sister-in-law, were arrested for immigration fraud in connection with a sham marriage between Marquez and Mariya Chernykh (the sister-in-law of Farook's brother). All three pleaded guilty.
The San Bernardino massacre was the deadliest mass shooting in California since the 1984 killings at McDonalds in San Ysidro.
—Online Sources