San Bernardino: the aftermath

The Federal Bureau of Investigation (FBI) on Thursday was investigating whether the worst mass shooting in the United States in three years was a case of terrorism or workplace violence.

Two assailants clad in black tactical clothing opened fire Wednesday on a holiday party at a social services center in San Bernardino, California, killing at least 14 people and wounding more than 20 others.

Police identified the perpetrators as Syed Rizwan Farook, 28, and Tashfeen Malik, his 27-year-old wife. Both died in a shootout with law enforcement after fleeing the scene in a black SUV. Farook and Malik were armed with two AR-15 style semi-automatic assault rifles as well as Llama and Smith & Wesson handguns.

"The two handguns were purchased by him, the rifles were not, but all four guns were legally purchased," Jarrod Burguan, the police chief of San Bernardino, told a press conference. "There's no criminal record that he had that we're aware of."

After the shootout, police found 1,400 rounds of .223 caliber ammunition and 200 rounds of 9mm ammunition. A sweep of the couple's home in Redlands, California turned up more than 4,000 rounds of ammunition as well as 12 pipe bombs and tools to construct explosive devices.

"Clearly they were equipped and could've continued to carry out another attack," Burguan said. "They came prepared to do what they did, as if they were on a mission," he said.

'No parallel in the world'

In the aftermath of the shooting, the public debate in the United States immediately turned to the epidemic of mass shootings that has gripped the nation.

The massacre in San Bernardino was the worst since 2012, when a lone gunman shot dead 20 children and six adult staff at Sandy Hook Elementary School in Newtown, Connecticut. President Barack Obama on Wednesday called for stricter gun laws in the aftermath of the tragedy in California.

"We have a pattern now of mass shootings in this country that has no parallel anywhere else in the world," Obama told CBS News. "There are some steps that we could take not to eliminate every one of these mass shootings, but to improve the odds that they don't happen as frequently."

'Rarely a singular motive'

There have been fears, just weeks after the attacks in Paris, that the massacre in San Bernardino was an act of Islamist terrorism.

Farook, an American of Pakistani descent, was described by a co-worker as a quiet man and devout Muslim who rarely talked about his religion, according to the Los Angeles Times.

The Times reported that Farook met his future wife, Malik, online and traveled last year to Saudi Arabia to meet her. Malik was a Pakistani citizen.

The couple targeted the holiday party of the county health department, Farook's employer of five years, which had rented space at the social services center. Investigators are examining workplace issues as a possible motivation for the shootings.

"Human beings are very complex animals, they rarely do something for a singular motive," Michael German, an expert on national security and civil liberties at the Brennan Center for Justice, told Deutsche Welle.

Guns, mental health and ideology

There have been 295 mass killings in the United States since 2006, approximately one every two weeks, according to an investigation by USA Today. The FBI defines a mass killing as taking four lives or more. Statistics change based on terminology and definitions.

Mass killings are routinely perpetrated by lone shooters with personal issues. Two cases had known or alleged connections to Islamist extremism.

In 2009, Nidal Hasan, an army psychiatrist, opened fire at Fort Hood, Texas, killing 13 people. Hasan was in contact with Anwar al-Awlaki, the imam and American citizen who was later killed in a 2011 US drone strike in Yemen. Washington said al-Awlaki was an al Qaeda recruiter.

Hasan said he attacked Fort Hood to defend the leaders of the Taliban. The military classified the shooting as an act of workplace violence, not terrorism. According to US broadcaster NPR, military officials had expressed concerned about Hasan's mental health prior to the mass shooting at Fort Hood.

"You're going to have cases in which a personal motivation - dissatisfaction with a job, anger at the co-workers - mixes with more political, ideological reasons, " Lorenzo Vidino, director of the Program on Extremism at George Washington University, told DW.

In July of 2015, Mohammad Abdulazeez, a 24-year-old engineer, opened fire at a military recruiting center and a navy operations support center, killing four marines and a sailor. The case was investigated as an act of terrorism, though the FBI found no connections to Islamic State. Abdulazeez had substance abuse issues and received treatment for depression.

There was also an attack in Garland, Texas last May that was claimed by Islamic State. Two gunmen opened fire outside of an event displaying images of the Islamic Prophet Muhammad. Both gunmen were shot dead by police. There were no other casualties.

Hasan bought his pistol legally. Abdulazeez bought some firearms legally and may have acquired others illegally, according to law enforcement. The FBI hasn't released information on the origins of the weapons in the Garland attack.

"It's America, it's easy to get guns," Lorenzo said. "Nothing is done on weapons."

'Treat all violence equally'

According to a June study by the New America Foundation, white supremacists and anti-government radicals have killed 48 people in the United States, while Islamist extremists have killed 26 since the September 11, 2001 terrorist attacks.

Last week, Robert Dear opened fire with a semi-automatic rifle at a Planned Parenthood Clinic, which provides health services to women, killing three people and wounding nine others. Dear was reportedly a self-professed Christian, staunchly opposed to abortion, described as disturbed and a recluse. Planned Parenthood performs abortions among a host of other medical procedures.

"Whether it's the act of a lone shooter, like the vast majority of mass shootings are, or the work of a white supremacist terrorist, or a left-wing terrorist, or a Muslim terrorist - that shouldn't really matter very much, but it seems to matter for everything in terms of what our response is," German said.

"We need to address all threats of violence equally and not emphasize one over another because that will end up creating flawed policies," he said.