Hundreds of Muslims gather for prayer vigil to denounce California

Police later killed the couple, Syed Rizwan Farook and his wife, Tashfeen Malik, in a shootout. Farook angrily left the party before returning with Malik, both of them heavily armed with assault rifles and semi-automatic handguns.

Four hours later and two miles away, the couple died in a furious gunbattle in which they fired 76 rounds, while 23 law officers unleashed about 380, police said.

The assault-style rifles were purchased by someone who is now under investigation, said San Bernardino City Police Chief Jarrod Burguan.

At the social service center, the couple left three rigged-together pipe bombs with a remote-control detonating device that apparently malfunctioned, and they had more than 1,600 rounds of ammunition left when police killed them in their rented SUV, Burguan said.

A follow-up search of their home in nearby Redlands, Burguan said, revealed a cache of 12 more pipe bombs, hundreds of tools for making more, and upwards of 3,000 additional rounds of ammunition. Once inside the conference room, he said "the situation was surreal". "At this point in time we don't know the motivation".

An intelligence source told NBC News Thursday that Farook had been in contact with multiple persons, foreign and domestic, via social media who may be linked to terrorism. "It's not like he got angry and came back and started deleting and destroying things", an official was quoted as saying by the New York Times.

The official cautioned that such contact by itself "doesn't mean that you're a terrorist". "It is also possible that this was workplace-related", said Obama, who ordered flags flown at half-staff after the tragedy.

Investigators are still looking for a motive behind the deadliest United States mass shooting since the Sandy Hook Elementary School massacre of the innocents on Dec 14, 2012 when a gunman shot 20 children and 6 adult staff members in Newtown Connecticut.

Lt Mike Madden said he and officers who arrived later saw dead bodies and had to pass injured people as they tried to "engage the shooters" on Wednesday. Two of the wounded remained in critical condition Thursday.

Nonetheless, police say, it appears that the couple put a great deal of planning into Wednesday's attack - a dynamic they say indicates some sort of a long-term motive.

"There was obviously a mission here", David Bowdich, the assistant Federal Bureau of Investigation director in charge of the Los Angeles office said, in the wake of the killings at a holiday party for county employees at a social services center. "When we entered, there was fresh gunpowder and the smell of gunpowder in the air, leading me to believe the shooters were still there", he said.

"My husband was very outspoken about ISIS and all of these radicalized Muslims", she said to the Times. However, a USA official confirmed to Fox News that the two met and became engaged after Farook traveled to Saudi Arabia in September 2013. "Who'd have ever thought that this would be a place that would get attacked?"

"It was extremely loud", Madden told reporters, describing the aftermath of Wednesday's mass shooting there. "It may be that there were others who helped in this attack, and if that's the case we need to worry about them as well". Two weeks ago, Farook and one of the co-workers he killed, 52-year-old Nicholas Thalasinos, had a heated conversation about Islam, according to Kuuleme Stephens, a friend of the victim.

"He had worked with [Farook]", Jennifer Thalasinos said.

On the surface, Farook and Malik lived the American dream, the kind of life that would be the envy of many.