The FBI said it's investigating the massacre in San Bernardino, California, that killed 14 and injured 21 as a terrorist attack. If that's proven, it would be the deadliest strike by Islamic extremists on American soil since Sept. 11, 2001. Here is what is known about the attack, shooters and investigation:
THE ATTACK: SHOOTERS OPEN FIRE ON A LUNCHEON
Some 85-90 county workers were gathered for a training session and holiday celebration in a conference room at the Inland Regional Center, a sprawling social services complex serving thousands of people with disabilities. Two shooters wearing combat gear burst through a side door and started spraying bullets.
As people dove for cover, alarms sounded and water began spraying from a sprinkler system hit during the attack. Within a few minutes, the shooters were gone, a homemade bomb that never detonated was left behind and screams filled the air.
With the killers on the loose, fear gripped the city and hundreds of officers searched frantically. Authorities were tipped that one of the shooters was Syed Farook, a 28-year-old county health inspector who had attended the gathering with many of his co-workers. Police went to Farook's home in nearby Redlands and saw a dark-colored SUV like the one spotted at the shooting scene.
When the SUV pulled away from the home, a chase ensued and dozens of rounds were fired at pursuing officers. Television news helicopters captured the final scene: the bullet-riddled SUV stopped in the middle of the highway. Two people were dead — Farook and his 29-year-old wife, Tashfeen Malik.
THE SHOOTERS: QUIET, RELIGIOUS
Farook, born in Chicago to Pakistani parents and raised in Southern California, met his wife online. She was from Pakistan but traveled to Saudi Arabia where her family lived. Malik arrived in the U.S. on a K-1 visa for fiancées in July 2014 and the couple married the following month in Riverside. They had a 6-month-old daughter.
Farook graduated from California State University, San Bernardino and worked for the county for four years as a restaurant inspector. Co-workers uniformly described him as quiet but friendly.
Attorneys for the Farook family said he was a loner. He attended a mosque daily in Riverside until two years ago, when he began going to another mosque in San Bernardino. He was a daily attendee there as well but suddenly stopped going three weeks before the shooting, according to brothers Nizaam Ali and Rahemaan Ali, who attend the mosque.
Very little is known about Malik. She studied pharmacy at Bahauddin Zakariya University in the Pakistani city of Multan in 2012, according to the university's vice chancellor. A maid who worked in the Multan home where Malik lived said she would travel back to Saudi Arabia to be with her family when school was out. During her time in Multan, her style of dress became more conservative over time.
Farook family attorneys have described her as "just as a housewife" who was soft-spoken and little known, even to the family and the mother-in-law who lived with her. Following religious tradition in their home, men and women would remain separated during social visits.
THE INVESTIGATION: FBI LOOKS AT EXTREMIST TIES
The FBI's announcement that the agency was investigating the shooting as a terror attack does not mean the agency has concluded Farook and Malik were terrorists — only that investigators have gathered enough preliminary information to move their investigation in that direction.
A U.S. law enforcement official told The Associated Press that Malik used an alias on Facebook to make a declaration of support for the Islamic State and its leader. But there is no sign anyone from the group communicated with her or provided any guidance for the attack. A Facebook official told AP the post went up at 11 a.m., around the time couple began their attack.
David Bowdich, assistant director of the FBI's Los Angeles office, said investigators determined the shooters attempted to destroy evidence, including crushing two cellphones and discarding them in a trash can. Authorities are trying to retrieve the data.
FBI Director James Comey noted the bureau's investigation so far has shown no evidence that the shooters were part of a larger group or members of a terror cell.
Farook family attorney David Chesley said the FBI has yet to generate hard evidence of a terror connection. "If the most evidence there is to any affiliation is a Facebook account under another person's name ... then that's hardly anything at all," he said.
THE VICTIMS: A REFLECTION OF AMERICA'S DIVERSITY
The 14 people killed were different in every way imaginable. Men and women. Younger and older. White, black, Hispanic and Asian. Single and married.
Yvette Velasco, 27, left behind three sisters and memories of her beautiful smile. Benetta Betbadal, 46, was born in Iran and came to the United States at age 18 to escape the persecution of Christians after the Iranian Revolution. She married a police officer and was the mother of three. Michael Wetzel, 47, was a fun-loving and dedicated father to his six children.
Aurora Banales Godoy, 26, worked as an office assistant and was mother of a toddler son. Nicholas Thalasinos, 52, was a Messianic Jew and passionately defended Israel, actively debating religion in online forums and in person.
Sierra Clayborn, 27, was friendly and outgoing, quick to offer an encouraging word and enthused about her work as a health inspector. Robert Adams, 40, married his high school sweetheart and was planning their young daughter's first trip to Disneyland.
Daniel Kaufman, 42, ran the coffee shop at the social services center where the shootings occurred and was a larger-than-life character. Isaac Amanios, 60, was originally from Eritrea and he and Farook sometimes spoke Arabic to other.
The 58-year-old Damien Means was the father of two grown daughters passionate about serving his community and a dedicated Catholic. Shannon Johnson, 45, was a college baseball player and ardent sports fan with a great sense of humor.
Harry Bowman, 46, loved the outdoors and doted on his two daughters. Juan Espinoza, 50, was born in Mexico, a quiet man and the father of two children.
Tin Nguyen, 31, was born in Vietnam and had been planning a wedding with her fiance, whose birthday was the day before she died.
This story has been corrected to say Syed Rizwan Farook worked for the San Bernardino Department of Public Health for four years.