San Bernardino: What makes this shooting different?

Staff and residents are led away from the site of the shooting.

San Bernardino in California remains in shock after a shooting at a disabled service centre, in which 14 died and 17 were injured. The motive is still unknown.

Mass shootings are a frequent occurrence in the US, yet this case has proved surprising for many, because it is different to nearly all the others.

Multiple attackers

The vast majority of mass shootings are perpetrated by a single individual. In some of the most recent and deadly attacks, carried out in Colorado, Roseberg, Chattanooga and Charleston, all of the attackers were men who were all described later as "loners" or alienated from others.

This time there are two suspects, Syed Rizwan Farook and his partner Tashfeen Malik. Farook was an American but Malik's nationality has not been confirmed.

Both were killed in a gunfight with police, after their black SUV was found in a residential area near Redlands in California.

A third suspect was detained after being seen running away from the scene. However, police have not confirmed whether that person is connected to the incident.

According to Shooting Tracker, a website that tallies gun violence in the US, there have only been eight confirmed cases of mass shootings being carried out by more than one attacker in the past year.

Woman involved

Female involvement is particularly rare, with 98% of mass shootings carried out by a man. This is the first mass shooting in over a year where a woman has been involved.

The last woman to have participated in such an attack was Amanda Miller, who was believed to be an accomplice to her husband Jerad during a shooting in Las Vegas last June. Five people, including the couple and two policemen, were killed.

On a larger scale, women are also less likely to own a gun. According to the Pew Research Center, 38% of men said they own a gun, rifle or pistol in their home, compared to only 31% of women.

Police brought in armoured vehicles while pursuing the shooters.

The attack was well planned

As police began to piece together the scene of the crime, it became increasingly unlikely that the incident was spontaneous.

The suspects were armed with two "long guns" - rifles or shotguns- and two semi-automatic handguns, and were wearing military-style clothing for the attack.

Police also found multiple explosive devices at the scene and law enforcement told the Los Angeles Times that pipe bombs had been thrown out of the SUV during police pursuit.

It was later discovered these were not bombs, but thick pipes enclosed in cloth to resemble a fuse. Additional items of concern were found in the car following the shootout.

The couple had also dropped off their six-month-old baby with relatives earlier on Wednesday.

Although no motive has been established, San Bernardino police chief Jarrod Burguan said that there "had to be some degree of planning" behind the attack, and the FBI have not ruled out terrorism.

Fleeing the scene

After carrying out an attack, the majority of mass shooters have turned their gun on themselves before being caught or taken down by police.

The FBI found that 40% of shooters killed themselves in incidences between 2000 and 2013, and that over 80% of these suicides occurred at the scene of the crime.

Farook and Malik chose to flee the scene after the shooting in a black SUV, leading to a police lockdown and manhunt of the surrounding area. Residents were told to remain indoors as police tracked down, pursued and eventually destroyed their car.

As they failed to leave the local area, it remains unclear what the individuals were planning - to escape, further attacks, or take their own lives elsewhere.

Explosives

Three explosive devices were reportedly found at the Inland Regional Center, the scene of the attack.

A spokeswoman for the Bureau of Tobacco, Firearms and Explosives, Meredith Davis, told NBC that the use of explosives adds to the impression that the attack was planned, recalling the Boston attacks as a rare example of where guns were used alongside explosives.

"I do recall in the Boston bombing that they threw devices, explosive devices at the police as they were being pursued," Davis said. "They planned that out very thoroughly, I would make a connection to that."

It is rare that mass shootings feature explosives alongside gun force.

However, after the Colorado attack on a Planned Parenthood facility police did make additional sweeps of the building as they were concerned Robert Dear, the attacker, had planted additional devices. None was found.