As soon as the first, frantic reports of a shooting came in, one detail above others worried police.
Witnesses said multiple people had stormed into a conference room at the Inland Regional Center in San Bernardino on Wednesday and opened fire.
The fact that more than one assailant appeared to have taken part in the massacre told law enforcement officials that something unusual was at play, even if they didn't know anything else about the shooting that left 14 people dead and more wounded.
In the scores of mass shootings that have occurred in the U.S. over the last 15 years, nearly all of them have involved an attacker acting alone.
A recent FBI analysis of 160 incidents, from 2000 to 2013 in which assailants were "actively engaged in killing or attempting to kill people," found that only two were carried out by two or more people working in tandem.
In 2011, a 22-year-old man and one or more unidentified shooters opened fire at a house party in Queens, N.Y. The man had been at the party earlier that night but left after arguing with others and returned shortly afterward.
A year later, two men fired handguns in the streets of Tulsa, Okla., killing three people and wounding two, according to the FBI analysis.
An earlier addition to the rare exceptions was the 1999 rampage at Columbine High School in Colorado, where two students killed 12 classmates and one teacher.
By nightfall Wednesday in San Bernardino, two suspects — a man and a woman — had been shot dead by police, and FBI agents were raiding an apartment thought to be connected to the shooters. Officials had yet to explain what might have motivated the attack at the Inland Regional Center.
Ron Avi Astor, a behavior health professor and mass shooting expert at USC, said the details of the shooting that had surfaced so far raised more questions than they answered.
On one hand, Astor said, reports that one of the shooters had attended a holiday gathering of county health workers at the conference hall and left after having an argument pointed to the possibility that the shooting was rooted in a workplace grudge.
But the preliminary information released by police that as many as three people could have taken part in the shooting didn't fit with the idea that the killings were part of an attempt to settle one man's scores, Astor said.
"You don't just go home to a friend or family member, hand them a rifle and say, 'Come help me,'" Astor said. "There had to be discussion and planning beforehand about carrying out something like this."
The question of whether the suspects were terrorists driven by religious or political ideology loomed over the investigation. David Bowdich, the region's top FBI official, said at a news conference Wednesday night that investigators had reason to believe it may be a factor.
That possibility further muddied the water, Astor said.
The assault rifles used, the coordinated nature of the attack and the shooters' ability to escape before police arrived pointed to a level of preparedness and professionalism that runs against a typical mass shooting scenario, Astor said.
And yet, he said, a holiday party for civil servants is not a target that carries the symbolic weight terrorists look for when planning their attacks.
"The target is a very soft target. The whole thing seems strange," he said. "What meaning does the place have and why kill so many people there?
"There's more to this story than what we know right now," he said.
Although it is rare to encounter multiple shooters, police train for the possibility, said John Incontro, police chief in San Marino and a former Los Angeles police captain who oversaw the department's SWAT unit.
After the 2008 terrorist attacks in Mumbai, India, which involved several teams of assailants hitting different locations simultaneously, police in the U.S. revamped their training to plan for such attacks.
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