SAN BERNARDINO (California) • On Wednesday morning, Syed Rizwan Farook and his wife Tashfeen Malik left their six-month-old daughter with his mother, telling her that they were going for a doctor's appointment, a relative said.

By nightfall, it was clear that was a ruse. Police said the couple spent the day carrying out a rampage at a social services centre that killed at least 14 people before leading officers on a chase that ended with the two dead in a gunfight in a suburban Californian neighbourhood.

Before the attack, Farook, 28, born in Illinois and whose parents are from Pakistan, joined his colleagues at an annual holiday party for the San Bernardino County Public Health Department, where he had been working for five years as an environmental inspector, officials said.

But he soon stormed out, before returning with Malik in a black sport utility vehicle. They spent "several minutes" shooting inside one of the buildings and then fled.

They also placed bombs at various spots, which police detonated.

The suspects wore masks and body armour, and were armed with assault rifles and handguns, said San Bernardino police chief Jarrod Burguan at a news conference.

"They were dressed and equipped in a way that indicates they were prepared. They came prepared to do what they did, as if they were on a mission," he said.

A third individual was detained while running from the scene of the shootout, but police said it was not immediately known whether that person was involved.

While the motive remained unclear, Mr Burguan and Mr David Bowdich, assistant director of the Federal Bureau of Investigation's office in Los Angeles, said they were not ruling out terrorism.

The attack in San Bernardino, 100km east of Los Angeles, was unlike nearly every other shooting of its type in the United States in the past 15 years because it involved more than one assailant, including a woman, and an apparently well-planned escape route.

It did not bear the hallmarks of a lone-wolf killing, yet differed from attacks like those perpetrated by the Islamic State in Iraq and Syria or other extremist groups, which focus on public places where carnage would have the biggest shock value and frighten residents the most, like the attacks in Paris last month.

At a news conference called by the Los Angeles area chapter of Muslim advocacy group Council on American-Islamic Relations, Mr Farhan Khan, Farook's brother-in-law, said he was bewildered by the news.

"Why would he do that? Why would he do something like this? I have absolutely no idea, I am in shock myself," said Mr Khan, who last spoke to Farook a week ago.

NEW YORK TIMES, REUTERS