As many as three gun-wielding ­assailants opened fire on a holiday party for county employees Wednesday, killing 14 people in the deadliest mass shooting in the United States since the Sandy Hook Elementary School massacre three years ago this month.

Hours after the shooting, law enforcement officials said two attackers — a man and a woman, neither identified by name — had been killed in a police shootout several miles from the site of the original attack. A third suspect who had been seen fleeing the scene was taken into custody, but police said they were not sure whether he had been one of the shooters.

An FBI official said at an evening news conference that authorities could not rule out the possibility that terrorism was the motive.

“One of the big questions that will come up repeatedly is: ‘Is this terrorism?’ ” said David Bow­dich, assistant director in charge of the FBI’s Los Angeles office. “It is a possibility. We are making some adjustments to our investigation. It is a possibility. But we don’t know that yet. And we are not willing to go down that road yet.”

Police in San Bernardino, Calif., said heavily armed gunmen killed 14 people and injured 17 others during a holiday party for county employees. Here's what we know about the mass shooting. (The Washington Post)

A senior U.S. law enforcement official identified one of the suspects as Syed Farook. Records show a Syed Farook works as a health inspector for the San Bernardino County Department of Public Health, which was hosting Wednesday’s holiday party.

The attack spawned a tense, confusing and terrifying day in Southern California as the shooters — their number unclear, their identities unknown, their motives unimaginable — fled the scene in a black SUV and eluded capture for hours.

Police followed a tip that led them to a house in the nearby city of Redlands; records show that a father and son named Syed Farook have lived at that address. During a stakeout there, they spotted an SUV that matched the description of the suspects’ vehicle, and when the occupants drove away, police gave chase. A law enforcement source said the suspects threw objects — possibly pipe bombs — out the window.

The suspects drove back into San Bernardino and, reaching a residential neighborhood, stopped and began exchanging gunfire with police. One officer was injured in the firefight, but police said the wounds were not life-threatening.

The two suspects were killed, their vehicle riddled with bullets.

The immediate aftermath was captured by helicopter new crews, who provided a live feed to a national audience watching on television and online. Police officers swarmed the neighborhood, guns drawn, taking cover behind walls and armored SWAT vehicles. Three armored vehicles surrounded the SUV, and officers slowly and methodically determined that no one inside had survived the hail of gunfire.

What we know about the shooting in San Bernardino, California

The two slain suspects were armed with assault rifles and handguns, San Bernardino Police Chief Jarrod Burguan said at an evening news conference.

At least 17 people were being treated in local hospitals for injuries suffered in the late-morning rampage. A spokeswoman for Loma Linda University Medical Center said two of the five patients treated there were in critical condition.

The Inland Regional Center is a three-building complex that houses a conference center and serves more than 30,000 people with developmental disabilities. The attack occurred in the conference center’s first-floor banquet room, where the public health department was hosting a holiday party, complete with Christmas trees and other decorations.

Witnesses reported seeing three people in black clothing using long guns akin to assault weapons. Recent mass shootings in the United States have typically involved a lone gunman, often someone mentally unstable or consumed with rage. Multiple-shooter events are extremely rare: According to a recent FBI report on 160 “active shooter incidents” between 2000 and 2013, all but two involved a single shooter.

“They came prepared to do what they did as if they were on a mission,” Burguan said. “They were dressed and equipped in a way that indicate they were prepared.”

Before the shooting, witnesses told police that one person left the holiday party after some kind of incident, Burguan said.

“Somebody did leave; there was some type of dispute or something when somebody left that party,” Burguan said. “But we have no idea if those are the people that came back.”

The San Bernardino fire department first responded to reports of gunfire at 10:59 a.m.

“Dad, I’m scared,” one woman who worked in the center and was taking shelter during the shooting wrote in a cellphone text to her father, Tom Carillo, according to a report in the San Bernardino Sun. She was later evacuated safely.

“People shot. In the office waiting for cops. Pray for us. I am locked in an office,” another woman wrote to her father, Terry Petit, according to the Associated Press.

Melinda Rivas, a social worker who works at the center, said she heard a woman shout, “There’s a shooting!” She and co-workers barricaded themselves in a conference room. She called her two adult children: “There’s a shooting going on. Be safe,” she told them.

“This is one of those things I’ve often seen on the news, and now I was a part of it,” Rivas said later in the day, having been evacuated by the SWAT team.

Chris Nwadike, an employee of the public health department, said the gunfire erupted while he was on a bathroom break from a meeting.

“We heard something like explosives — big sounds first, then a few seconds, then we heard the gunshots,” he said.

Someone in the bathroom said, “Everybody lie down,” Nwadike recalled. After about 10 minutes, the police came and directed everyone out of the building.

The shooting shut down much of Southern California’s “Inland Empire,” which stretches into the desert east of Los Angeles. Schools and county buildings were put on lockdown for fear that gunmen were on the loose and prepared to attack again. The Federal Aviation Administration imposed temporary flight restrictions over the city at the request of local law enforcement.

SWAT team members went room by room through the three buildings in the IRC complex. Numerous victims were seen in television footage being carried away on stretchers. Medical teams set up a triage area on a nearby public golf course.

The center employs about 670 staff members, according to the organization’s Facebook page. The center advertises its three core values as “independence, inclusion, and empowerment” and says that it is committed to eliminating barriers for individuals with developmental disabilities so that they can “live a typical lifestyle.” The center held its own holiday party Tuesday, and a brief video clip showed staffers and clients in wheelchairs dancing to the 1980 mega-hit “Celebration” by Kool & the Gang.

“We don’t know who the gunmen are, or why this happened. It’s devastating,” said Marybeth Feild, president of the Inland Regional Center’s board of directors, who was not in the building at the time of the shooting. “I just don’t know how we’re going to recover from this. It’s just overwhelming. Why would anyone target a social service center?”

By mid-afternoon, police were bringing relatives of victims to the Rudy C. Hernandez Community Center.

“I’m worried, petrified, scared,” said Sherry Esquerra, whose daughter and son-in-law, Shawna and Daniel Timmons, work at the IRC. She said she’d left phone messages but hadn’t heard back.

“She would text me. I know she would know that I am worried,” Esquerra said. “They do social work. I don’t know how anyone could do this to them.”

At least 100 family members could be seen sitting in the bleachers at the community center, where authorities assembled clergy and grief counselors.

Meanwhile, police were locking down or evacuating stores and restaurants miles from the scene of the shooting. Kim Scott, a cashier at a 99 Cents store, said police told workers and customers to evacuate from the Waterman Discount Mall and surrounding stores.

“It’s scary. We’re supposed to be evacuating,” she said. “I’m behind Register 3. We’re hiding out until they tell us to get out. . . . I want to go home. I don’t know what’s going on.”

The San Bernardino massacre follows a series of mass shootings just in the past six months. They include attacks on a black church in Charleston, S.C.; at a military recruitment center in Chattanooga, Tenn.; in a movie theater in Lafayette, La.; at a community college in Oregon; and just last week at a Planned Parenthood facility in Colorado Springs.

One Internet site that tracks mass shootings — defined as events in which four or more people are killed or injured — reported that San Bernardino’s tragedy was the 355th such shooting this year, a pace of more than one a day.

“Obviously our hearts go out to the victims and the families,” President Obama said in a television interview. “The one thing we do know is we have a pattern now of mass shootings in the country that has no parallel anywhere else in the world.”

Obama called for “common sense” gun-safety laws. And he noted that while people are fearful of a terrorist attack, under current law, people on the U.S. no-fly list can still legally buy a gun.

“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 them,” Obama said.

Earlier in the day, even before the shooting, doctors in white lab coats descended on Capitol Hill to ask Congress to lift a decades-old funding ban on research on gun violence.

Most of the presidential candidates took to Twitter to respond to the massacre, with Republicans and Democrats taking different tacks. The GOP contenders almost universally offered prayers for the victims.

“California shooting looks very bad. Good luck to law enforcement and God bless. This is when our police are so appreciated!” tweeted Republican front-runner Donald Trump.

The three major contenders in the Democratic race, meanwhile, specifically cited the need to stop gun violence.

“I refuse to accept this as normal,” tweeted Democratic front-runner Hillary Clinton. “We must take action to stop gun violence now.”

Dauber is a freelance writer. Berman and Achenbach reported from Washington. Freelance writer Martha Groves in San Bernardino and staff writers Adam Goldman, Lindsey Bever, Niraj Chokshi, Ann Gerhart, Sari Horwitz, Elahe Izadi, Wesley Lowery, Kevin Sullivan, Julie Tate and Alice Crites in Washington contributed to this report.