SAN BERNARDINO, CALIF. —Dressed in black face masks and tactical gear, armed with long guns and pistols, they entered a holiday party for county health workers as it was in full swing. Before they fled, they had killed 14 people and wounded 17.

Four hours later, as fearful residents were ordered to stay home and scores of officers swarmed the streets, authorities chased a black SUV carrying two suspects from a Redlands home. As TV news stations broadcast live overhead, authorities and the suspects traded gunfire.

When it was over, a man and woman connected to the assault were dead. One body lay in the street, blood pooling. Another was recovered from the vehicle. A police officer was also wounded in the firefight but is expected to survive.

The motive for the shooting was unclear.

“Is this a terrorist incident? We do not know,” said David Bowdich, assistant director of the FBI’s Los Angeles field office. He later said terrorism had not been ruled out as a motive.

The identities of the man and woman killed by police were not immediately released.

A senior federal official monitoring the case said investigators believe one of the shooters left the party after getting into an argument and returned with one or two armed companions. Local officials at an evening news conference said it was not clear whether the people involved in the dispute were the same people involved in the shooting.

Law enforcement sources identified one of the suspects as Syed Farook. Two separate sources said one of the suspects was an American citizen, but did not specifically name Farook.

Public records show a person named Syed R. Farook was employed by the San Bernardino County Health Department as an environmental health specialist, but it was not clear if that was the same person involved in the shooting. The party was a gathering of employees of the health department.

A third person was detained in the area where the pursuit ended, but authorities said they had not connected the person to the shooting. They raised the possibility that there could be at least one other person involved in the shooting and additional people involved in the planning.

A federal law enforcement source told the Los Angeles Times that the suspects hurled what were believed to be pipe bombs at police during the vehicle pursuit. Burguan said police recovered one device, but it turned out not to be an explosive.

The shooting Wednesday was a grim marker: It was the deadliest mass shooting since the massacre of 20 children and six teachers at Sandy Hook Elementary School in Newtown, Conn., in 2012.

People at the party described a terrifying scene.

“Everyone dropped to the floor,” Denise Peraza, 27, told her relatives. “The guys opened fire for 30 seconds, randomly, then paused to reload and began firing again.”

Peraza was hiding under a desk when she was struck in the lower back. After the attackers left, the scene was silent for about five minutes. Then the doors swung open again, and a swarm of police officers entered the room.

The first 911 call came at 11 a.m. local time.

As it has in America’s never-ending string of mass shootings, word spread via social media. The first sign that something horrifying was underway came when the San Bernardino County Fire Department reported via Twitter that the agency was responding to a scene of “20 victim shooting.”

The attack at the Inland Regional Center, which serves developmentally disabled people, rippled across San Bernardino, a city located about 100 kilometres east of Los Angeles. All county schools, as well as city government buildings and courthouses, were on lockdown as police continued to search for the assailants. School officials, however, stressed that students had not been in danger and were dismissed on the regular schedule.

During a news briefing, San Bernardino Police Chief Jarrod Burguan said information about the party being the focus of the attack was “preliminary” and he declined to comment on a motive for the shooting. He did confirm that one person at the party left before the shooting, after an argument, but said it wasn’t clear if that person was one of the eventual shooters.

“We have no information at this point to indicate that this is terrorist related, in the traditional sense that people may be thinking,” Burguan said. “Obviously, at a minimum, we have a domestic terrorist-type situation that occurred here.”

Hundreds of people were on the grounds at the time of the attack, Burguan said. Inland Regional Center officials said the conference room where the party was taking place can hold up to 200 people.

Chaos followed the gunfire. At first, some at the scene mistook the shots and law enforcement response for a routine disaster drill.

Dorothy Vong, a nurse who was working in a nearby building, captured the tension in a video.

As law enforcement officials sprinted toward the scene, someone can be heard saying, “Oh, that is scary.”

“They’re all geared up!” someone else says. “Rifles and everything!”

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In the background, someone laughs. Then the reality sets in.

Carlos Ortiz’s son Kevin Ortiz was shot twice in the leg and once in the shoulder.

Carlos Ortiz, 54, was among a dozen people holding hands in a prayer circle outside Arrowhead Regional Medical Center, where numerous victims were taken.

“Kevin called me immediately after he got shot and said, ‘I’ve been shot three times, Dad. I’m in pain. Don’t worry. There’s a policeman with me.’ ”

Seconds later, the phone call ended.

Fred Henning was holed up inside the paralegal’s office where he works with his wife, about a block from the scene of the shooting. Henning said they were standing outside as helicopters swooped overhead.

“We just came inside because it could be stray bullets, who knows?” Henning said.

The block where the shooting took place is home to a number of businesses, Henning said, including a three-building complex that houses his office and about 140 others.

Lynn Spicer, an employee at West Tech/Webcop Interactive Systems, inside a nearby office building, said police were not allowing anyone to leave the area.

“I just heard sirens all day, and I went out and I saw nothing but massive cops were out,” Spicer said.

Spicer said more than 100 people were brought over from the Inland Regional building and gathered in her office complex’s parking lot.

“It’s shocking that it’s right across the way,” Spicer said. “It’s very scary.”

The president was being interviewed by CBS on Wednesday morning when news of the attack broke, and said the repeated occurrence of mass shootings shows the need for stricter gun laws in the U.S.

“Obviously, our hearts go out to the victims and the families,” he said. “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.”

As night fell, some were still left to wonder if their loved ones had been wounded, or even killed, in the morning attack. In East Hollywood, Mindy Velasco paced around her office, repeatedly calling area hospitals, trying to find her niece, who worked for the county health department.

Yvette Velasco, who is in her late 20s, had told her family she planned to attend the holiday banquet.

“I’m fearing the worst,” said her aunt, her voice breaking. “Her sister asked me to pray because we know she was there.”

As Mindy worked the phones, Yvette’s father visited local police stations and evacuation centres in search of information to no avail.

“Yvette is usually in constant touch with us. She would definitely be in contact after something like this,” she said. “But, as of now, there is nothing.”