One of the first officers on the scene of Wednesday's mass shooting in San Bernardino, Calif., said he saw "pure panic" on the faces of the injured as police entered the area where the attack took place.

"It was unspeakable, the carnage we were seeing," Lt. Mike Madden told reporters at a Thursday evening news conference.

Madden, a 24-year veteran of the force, was on a lunch break when the first report of an active shooter came in around 11 a.m. PT Wednesday. About a kilometre away, he rushed to the Inland Regional Center.

Candlelight vigil held Thursday night to remember the 14 people killed 3:43

He described victims moaning and fire alarms going off in a conference room where the county health department was holding a holiday luncheon.

Madden said although it was something police train for, especially in the wake of the 1999 Columbine school shootings, "it's something you're never actually prepared for."

A candlelight vigil for the victim's of Wednesday's shooting took place Thursday night at a baseball stadium in San Bernardino, Calif. (Mario Anzuoni/Reuters)

Madden described the scene inside the centre as "surreal" and said it was immediately evident that the reports of multiple dead and injured were "100 per cent true."

He said the first four officers who entered the building had to make the difficult choice to not attend to injured individuals but continue moving through the building.

"Our goal had to be to locate the shooters and deal with them before we could get further assistance to those in need of medical attention," Madden said.

This was a tragedy I have never experienced in my career. - Lt. Mike Madden, first officer on scene

"This was a tragedy I have never experienced in my career," he added. "But everyone who responded knew we had to bring calm to the chaos that was going on."

Earlier Thursday, authorities described how Syed Rizwan Farook and his wife, Tashfeen Malik entered the Inland Regional Center wearing black tactical gear and carrying hundreds of rounds of ammunition.

Police said they entered the area where the staff party was going on and "sprayed the room with bullets," firing 65 to 75 rounds during an attack that authorities described as a carefully planned "mission."

Farook, 28, and Malik, 27, had more than 1,600 rounds of additional ammunition in the SUV they arrived in and another 3,000 rounds, 12 pipe bombs and bomb-making materials at their house in nearby Redlands, Calif., police said.

The black SUV rented by the couple who carried out Wednesday's attack is shown after an intense shootout between police and the two attackers. Police fired 380 rounds of ammunition during the gun battle, which left the two shooters dead. (Mario Anzuoni/Reuters)

The couple died about four hours after the attack in a furious shootout with police that saw officers unleash about 380 rounds of ammunition into the couple's black, rented SUV.

The two seemed to have targeted a holiday work party for employees of the San Bernardino County Department of Public Health, who had gathered in a rented-out space at the centre after a morning training session. About 75 to 80 people were at the event at the time of the shooting.

Farook worked for the department as an environmental health specialist and had earlier attended the event but slipped out at some point before returning with his wife.

Police were processing three different crime scenes related to the shooting at the Inland Regional Center. (Mike Blake/Reuters)

Earlier Thursday, San Bernardino Police Chief Jarrod Burguan offered a grim morning-after inventory that suggested Wednesday's bloodbath could have been far worse.

At the social services centre, the couple left behind an explosive device — three rigged-together pipe bombs with a remote-control detonating device that apparently malfunctioned. Combined with the bombs and ammunition found at the couple's rented home in Redlands, it seemed to point to premeditation.

A bad look back at a problem that is picking up speed 3:26

"I think that based upon what we have seen and based upon how they were equipped, there had to have been some degree of planning that went into this," Burguan said.

"There was obviously a mission here. We know that," David Bowdich, assistant director of the FBI's Los Angeles office, said as the bureau took over the investigation.

"We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately."

Motive unclear

Combing through at least three crime scenes, investigators are still trying to piece together a motive, whether it be terrorism, a workplace grudge or some combination of motives.

Fourteen people, ranging in age from 26 to 60, were killed and 21 people were wounded. At the evening news conference, officials said 12 of the deceased and 18 of the wounded were county employees.

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It was the deadliest mass shooting in the U.S. since the attack three years ago at Sandy Hook Elementary School in Newtown, Conn., that left 26 children and adults dead.

According to unnamed U.S. intelligence officials, Farook was in contact via social media with a terrorism suspect being investigated by the FBI, but he and his wife were not under investigation.

Speaking to The Associated Press, one official said that Farook's contacts online did not involve "significant players on our radar" and dated back some time.

Suspect recently travelled outside U.S.

Farook was born in Chicago and raised in Southern California while Malik had a Pakistani passport.

Syed Rizwan Farook worked for the San Bernardino County Department of Public Health and is believed to have left a department holiday party abruptly on Wednesday and returned with his wife to carry out the shooting. (California Department of Motor Vehicles /Associated Press) Relatives said Farook had travelled to Saudi Arabia in July 2014 to meet his wife, also making a stop in his family's native Pakistan. The couple reportedly first met online, and Malik entered the U.S. on a fiancée visa.

The pair also had a six-month-old daughter, who was left in the care of relatives Wednesday morning.

Farook's family is Muslim and originally from Pakistan, according to Hussam Ayloush, executive director of the Council on American-Islamic Relations, which organized a press conference Thursday morning attended by Farook's brother-in-law Farhan Khan.

Malik was born in Pakistan and lived in Saudi Arabia until they married, Ayloush said.

He was a very quiet person, peaceful, never had an argument with anyone. - Source

The director of the Islamic Center of Riverside, a mosque Farook attended regularly, described him as a devout Muslim who made the pilgrimage to Mecca in Saudi Arabia a few years ago and celebrated his wedding reception at the mosque.

"His degree of faith is very high," the director, Mustafa Kuko, told Reuters. "He was a very quiet person, peaceful, never had an argument with anyone or a dispute."

Kuko said Farook attended morning and evening prayers regularly from 2012 to 2014, when he stopped coming abruptly.

Farook worked at health dept. for 5 years

Before Wednesday's attack, Farook had worked for the county health department for five years, inspecting restaurants, bakeries and public swimming pools, according to the Los Angeles Times.

The newspaper said his duties were to check chlorine levels, screen handwashing facilities and make sure food surfaces were clean. State records indicate he is registered as an environmental health specialist.

All 14 victims of the shooting have been identified and authorities released their names Thursday. (Jae C. Hong/AP)

Co-worker Patrick Baccari said he was sitting at the same table as Farook, who suddenly disappeared Wednesday, leaving his coat on his chair. The New York Times reported that he left angry after some kind of dispute.

Baccari described Farook as reserved and said he showed no signs of unusual behaviour. Other media reports said he was well liked by his colleagues.

If I hadn't been in the bathroom, I'd probably be lying dead on the floor. - Patrick Baccari , co-worker of Syed Rizwan Farook

Baccari said he stepped into the bathroom when the shooting started around 11 a.m. PT Wednesday and suffered minor wounds from shrapnel slicing through the wall.

"If I hadn't been in the bathroom, I'd probably be lying dead on the floor," he said.

Khan, who is married to Farook's sister, told reporters he last spoke to his brother-in-law about a week ago. He said he was in shock, condemned the violence and had "absolutely no idea why he would do this."

Obama says shooting could be terrorism-related

U.S. President Barack Obama said it's possible that Wednesday's shooting was related to terrorism but cautioned that authorities still don't know the exact motive.

CBC's L.A. correspondent Kim Brunhuber speaks to shocked residents 0:58

Obama said Thursday he spoke with San Bernardino Mayor Carey Davis and thanked him and his team for managing "an extraordinarily difficult situation."

He said the nation must make it harder for people to carry out violence, adding, "we all have a part to play."

But he acknowledged the threat can't be eliminated completely. He said it will be important for all Americans, including state legislators, to see what they can do.