The US authorities are leaving open the possibility that a mass shooting an hour outside Los Angeles was an act of terror, as the country reels from what is its deadliest episode of gun violence since the 2012 attack at an elementary school in Sandy Hook on the east coast.
Two shooters are said to have attacked a social services centre in San Bernardino, California, on Wednesday morning, killing 14 people and injuring 17 others.
In the intense manhunt that followed, two suspects - a man and a woman wearing assault-style clothing - were reported killed by police.
They were identified as Syed Rizwan Farook, 28, and Tashfeen Malik, 27. The couple were said to be married with a six-month-old daughter.
Syed Farook was an inspector with the San Bernardino County Public Health Department. Health department employees were having a Christmas party in the building at the time the shooting took place. He was an American citizen.
In the immediate aftermath of the massacre, theories as to the motives behind it ranged from domestic terrorism to the suspect being disgruntled at work.
A Federal Bureau of Investigation agent said it was too early to rule out the possibility of terrorism.
"I know one of the big questions that will come up repeatedly is, 'Is this terrorism?'" said Mr David Bowdich, assistant director in charge of the FBI's Los Angeles Field Office.
"I am still not willing to say that we know that for sure. We are definitely making some movements that it is a possibility. We are making some adjustments to our investigation. It is a possibility, but we don't know that yet."
There are currently no clear links between any of the alleged assailants and the Islamic State in Iraq and Syria, and the militant group has not claimed responsibility.
Even though gun violence is becoming increasingly common in the United States, mass shootings involving more than one attacker are extremely rare.
According to an FBI report released last year, there were only two "active shooter incidents" involving more than one person in the 170 such incidents that took place between 2000 and 2013.
The attack began at around 11am in San Bernardino county when the shooters stormed the Inland Regional Centre, an agency that provides social services to the largely working-class city. The assailants then fled in a black sport utility vehicle, leaving behind a number of explosive devices.
"These are people that came prepared. They were dressed in a way to indicate they were prepared. They had long guns, not handguns," San Bernardino police chief Jarrod Burguan told reporters.
For much of Wednesday, the city of San Bernardino was on lockdown, with residents told to stay indoors and public offices closed. And though schools and public offices were expected to be open yesterday, San Bernardino Mayor Carey Davis said the city remains on high alert.
Late on Wednesday, the Greater Los Angeles Area chapter of the Council on American-Islamic Relations (Cair) and the Islamic Shura Council of Southern California held a joint press conference to condemn the attacks and offer their condolences.
"The Muslim community stands shoulder to shoulder with our fellow Americans in repudiating any twisted mindset that would claim to justify such sickening acts of violence," said Cair executive director for the area Hussam Ayloush.
President Barack Obama, meanwhile, renewed his call for tighter gun control laws.
He said at an interview: "The one thing we do know is that we have a pattern now of mass shootings in this country that has no parallel anywhere else in the world. We should never think that this is something that just happens in the ordinary course of events because it doesn't happen with the same frequency in other countries."