Fourteen people were killed and 21 were wounded when Syed Rizwan Farook, a 28-year-old county health inspector, and his wife, 27-year-old Tashfeen Malik, opened fire at Farook’s office holiday banquet held at the Inland Regional Center, a state-run social services facility in San Bernardino, California, late Wednesday morning. The rampage, which ended with the couple dead, is the deadliest shooting since the Newtown massacre nearly three years ago and the fifth deadliest mass shooting in United States history.

The New York Times, citing San Bernardino Police Chief Jarrod Burguan, reports that Farook left the party “‘angry’ after a dispute of some sort,” then “returned with Ms. Malik around 11 a.m. — heavily armed.”

Burguan said Farook and Malik, who donned vests and masks after dropping off their 6-month-old daughter with Farook’s mother, burst into a conference room just after 11 a.m. and “came prepared to do what they did, as if they were on a mission.” The Times adds that the couple wielded .223-caliber assault rifles and semiautomatic handguns and fled in a black SUV, leaving three explosive devices behind.

The mass shooting gave way to a manhunt in sprawling San Bernardino County (which is larger than nine states) that culminated in a broad-daylight shootout with police several hours later in a residential neighborhood two miles west of the crime scene. News helicopters hovered as a body lay in the road near a pool of blood and SWAT teams pulled a second body from the backseat of a black SUV riddled with bullets.

One officer was grazed by a bullet during the firefight but not seriously wounded. A third, unidentified person has been detained by law enforcement. Several hours later, police raided an apartment in Redlands as part of their investigation.

“Just another day in the United States of America,” began a report last night on the BBC. “Another day of gunfire, panic and fear.” Below, seven key pieces of context to keep in mind as more details emerge.

San Bernardino has struggled with gun violence for years

Today’s mass shooting in San Bernardino is the most high-profile incident of gun violence in a city that has struggled with the issue for years. Local residents lamented this August that city officials have done little to stem the problem.

Lieutenant Rich Lawhead of the San Bernardino Police Department believes that a confluence of factors is driving the city’s gun violence. “This is kind of a perfect storm right now,” he told the San Bernardino Sun in July 2014.

Amid a push to reduce California’s prison population, the city has seen an influx of recently released ex-convicts. And as Los Angeles cracks down on its gang problem, many of those trying to avoid law enforcement have relocated their operations to desert cities like San Bernardino. At the same time, the city’s police force has been slashed because of budget problems, leaving just more than 300 officers to patrol a city of 200,000 people. The combination of a growing criminal population, illegal gang activity, and dwindling law enforcement ranks has led to an uptick in gun violence, according to Lawhead.

“They came prepared to do what they did, as if they were on a mission.”

Jarrod Burguan, San Bernardino Police Chief

The city had 46 homicides in 2013, and 30 homicides in 2014. Most of the victims were connected with gangs or illegal drug activity. The Gun Violence Archive has recorded 37 instances of gun violence in San Bernardino this year alone, with 13 fatalities and 24 injuries. From 2006 to 2010, the city’s gun homicide rate was twice as high as San Diego’s. In her book Ghettoside, Los Angeles Times crime correspondent Jill Leovy notes that in San Bernardino County, the homicide rates for young black men are 20 times the national average.

The involvement of more than one shooter marks this event as an anomaly

In the vast majority of cases, mass shooters act on their own. An FBI study of active shooter incidents from 2000 to 2013 found that in 160 shootings, only two involved more than one gunman, with Columbine one of the exceptions. The other is another husband-and-wife team, Jerad and Amanda Miller, a disaffected couple who opened fire in a Las Vegas, Nevada, Walmart in 2014, killing two police officers and a concealed carrier who tried to intervene.

Another unusual aspect of this mass shooting is that the perpetrators fled the scene: The FBI study found that in only 25 out of 160 active shootings did the perpetrator manage to leave the area before police arrived.

The attackers’ motives are murky

Because the shooting targeted government workers and followed so closely on the heels of a shooting at a Planned Parenthood in Colorado Springs, Colorado, that may or may not have been politically motivated — an attack that reinvigorated a long-simmering debate over whether rampages perpetrated by lone American-born gunmen qualify as their own form of “domestic terrorism” — speculation quickly began regarding the assailants’ objectives. “There had to be some degree of planning that went into this,” Chief Burguan told reporters. “I don’t think they just ran home and put on these tactical clothes.”

At a press conference several hours before the attackers’ names were revealed, FBI Assistant Director David Bowdich told reporters:

“I know one of the big questions that will come up repeatedly is ‘Is this terrorism?’ And I am still not willing to say that we know for sure. We are definitely making some movements that it is a possibility. We are making some adjustments to our investigation. It is a possibility, but we don’t know that yet.”

When one of the gunmen was revealed to be Syed Farook — a U.S. citizen and pious Muslim who was born in Illinois, and living in the nearby city of Riverside — speculation shifted to Islamic extremism. Harry Houck, a law enforcement analyst at CNN, said that the FBI’s dominance of the crime scene made him think it was a terrorist attack.

But Casey Jordan, a criminologist, said this was most likely the work of a disgruntled employee, an assessment bolstered by witness accounts of Farook storming out in anger before returning and opening fire. Sources told NPR’s Dina Temple-Raston that party attendees “appeared to recognize his voice and build even though he was wearing a ski mask.”

Farhan Kahn, who is married to Farook’s sister, appeared at a news conference flanked by officials from the Council on American-Islamic Relations in Anaheim Wednesday night and said he had no idea why Farook would perpetrate a mass shooting. “I’m in shock myself,” he said.

The massacre was the 355th mass shooting of 2015

That’s according to the Mass Shooting Tracker, a crowdsourced database of incidents in which four or more people are shot. A mass shooting earlier in the day in Savannah, Georgia, resulted in a woman dead and three men injured.

Many media outlets (including The Trace) have shied away from using the mass shooting label for every incident with four or more injured, but this year’s run of high-profile attacks has changed that. The front page of this morning’s New York Times carries an article with this headline: “On Average, Mass Shootings Occur Daily in the U.S.”

As noted above, the San Bernardino shooting is the fifth deadliest in U.S. history. The 2007 Virginia Tech shooting claimed the most lives, with 32 dead. That massacre is followed by the 2012 Sandy Hook shooting, where 26 died; the Killeen, Texas shooting at a Luby’s restaurant in 1991, where 22 died; and a 1984 shooting at a McDonald’s in San Ysidro, California, where 21 died. A 1986 shooting at a post office in Edmond, Oklahoma, also resulted in 14 deaths.

Californians may soon vote on a set of ballot initiatives that would impose tough new gun restrictions

Wednesday’s shooting comes as California Lieutenant Governor Gavin Newsom is gathering signatures for a ballot initiative that would seek to reduce gun violence with a range of policies. The initiative would ban not only the sale and transfer but also the possession of magazines that hold more than 10 rounds of ammunition. If enacted, that prohibition would effect many of the magazines used by the type of rifles (some reports have described them as AR-15s) that the San Bernardino shooters carried.

Newsom is also calling for retailers of ammunition to acquire federal licenses (much as licensed gun dealers do); mandatory reporting of lost or stolen firearms; and a better system to recover guns from those prohibited from possessing firearms because of certain criminal records.

A Bystander Brought His Gun Within Yards of the Shooting But Chose Not to Try to Intervene

Glenn Willwirth owns a paper company across the street from the Inland Regional Center. When a man ran into his business and said a shooting was taking place, Willwirth grabbed his .45 caliber handgun and walked toward the center, taking position behind a water truck that was refilling a fire hydrant. Willwirth told CNN that he was able to draw a bead on the shooters but chose not to fire because of the chaos. In a separate interview with the Boston Herald, he added this about why he held back:

“I wasn’t going to travel onto their property to get involved — just because it’s not my property — but I went up to the edge of it and that was as close as I really needed to be getting to the whole situation. If they came at me it would have been a different story.”

President Barack Obama’s tally of post-mass-shooting remarks has reached 17

In an interview with CBS from the climate conference in Paris, President Obama delivered his 17th set of remarks following a mass shooting in the U.S.:

“The one thing we do know is that we have a pattern now of mass shootings in this country that has no parallel anywhere else in the world. And there’s some steps we can take not to eliminate every one of these mass shootings but to improve the odds that they don’t happen as frequently. Common sense gun safety laws, stronger background checks — and for those who are concerned about terrorism, some may be aware of the fact that we have a no-fly list where people can’t get on planes, but those same people who we don’t allow to fly could go into a store right now in the United States and buy a firearm, and there’s nothing that we can do to stop him. That’s a law that needs to be changed.”

House Speaker Paul Ryan also addressed the shooting as he spoke at the Capitol during a Christmas Tree lighting ceremony. After noting that the warm November weather in DC compared to a “nice night in July” in Alaska, where the tree was harvested, Ryan asked for a moment of silence in honor of the shooting’s victims. Then he discussed the tree.

[Photo: Gina Ferazzi / Los Angeles Times via Getty Images]