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Officials have told NBC News that it was likely a man called Syed Farooq and a woman, who one person described as his fiancee, were involved in the attack on a San Bernardino social services facility on Wednesday.
A third person who had been sought following the attack now appeared less likely to have been involved, the sources told NBC.
Up to three attackers opened fire at the Inland Regional Center facility, killing at least 14 people and wounding as many as 17, before fleeing the scene in a black SUV.
The hunt for the black SUV led police to an address in the neighboring town of Redlands, and police chased a vehicle that was seen leaving that address to travel back to San Bernardino. San Bernardino Police Chief Jarrod Burguan said the chase ended with officers engaging in a gun battle with a man and a woman, both of whom were armed with assault rifles and hand guns.The man and the woman were killed in the shootout.
A police officer was been injured in the gun fight but the injuries were not life threatening, the police chief said.
A third person was seen running from the vehicle and had been detained, Burguan said, adding in reference to the third person that "we do not know that they were involved" in the attack at the Inland Regional Center facility in San Bernardino.
Burguan said the two suspect were dressed in dark, "assault-style" clothing and that there was "sensitive stuff" around the vehicle, which police were searching to ensure there were no explosives inside. There were also reports that an explosive device had been found at the San Bernardino facility during a police sweep after the attack.
Multiple sources from multiple agencies identified one of the three attackers at the facility to NBC as Syed Farook. A person named Syed Farook is listed in public records as a resident of the Redlands address from which police tracked the SUV involved in the shootout, NBC reported.
David Bowdich, head of the Los Angeles FBI office, said federal agents and local law enforcement were being cautious about entering the Relands house because of concerns about explosives that might have been left there.
Reports first surfaced around 11:15 a.m. PT of a shooting near the Inland Regional Center.
At an early press conference after the attack, Police Chief Burguan said that the shooters were armed with long guns — not hand guns — and that preliminary information indicated they were ready for the attack. "They came prepared to do what they did, as if they were on a mission," he said.
If the death toll is correct, the San Bernardino attack is one of the deadliest mass shootings in modern American history — the worst since 27 were killed in Newtown, Connecticut, in 2012, according to NBC News.
So far in 2015, there have been more than 350 shootings in which four or more people were wounded, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of U.S. gun violence.
Both Burguan and Bowdich cautioned against describing the attack on the facility as an act of terrorism.
Burguan declined to characterize the attack as an act of "traditional" terrorism, but he said "obviously at minimumwe have a domestic terrorist-type situation."
Bowdich said that "we're definitely making some movements that it is a possibility [of terrorism] but we don't know that yet."
Reuters reported that the Muslim advocacy group CAIR said it condemned the shooting and a relative of a suspect named in media reports will take part in an evening news conference.
Reuters also reported that a man who claimed to be the brother-in-law of one of the suspected attackers had said that he had no idea what motivated the assault on the facility for people with developmental disabilities. Karhan Khan, who claimed his sister was married to one of the people suspected of carrying out the attack - he did not say which person - said he was "in shock."
"Why would he do that, why would he do something like this," Khan said at a news conference in Anaheim, California, according to the Reuters report. "I have absolutely no idea."
Terry Petit said he got a text from his daughter saying she was hiding after gunfire erupted at the social services facility where she works. Petit read the texts for reporters outside Inland Regional Center in San Bernardino, the AP reported.
He said she wrote: "People shot. In the office waiting for cops. Pray for us. I am locked in an office."
Media outlets reported that the shooting occurred after a dispute between people attending a holiday party at the facility.
Burguan confirmed that there was "some kind of dispute where someone left the party" but said police did not know if the attackers were the people involved in the dispute.
Marybeth Feild, the president and CEO of the Inland Regional Center, said the focus was on a building that housed at least 25 employees as well as a library and conference center.
"The incident is in the conference area" that an outside group was renting Wednesday, she said, adding that she was not at the center — which serves residents of San Bernardino and Riverside — and did not know at the time what outside group had rented the center. Feild said, however, that people served by the center also would have been in the building.
Lavinia Johnson, the center's executive director, told Reuters that a county personnel holiday party, possibly for the Department of Health, had been scheduled at the conference area.
A police spokeswoman told Reuters the center had been hosting an event when more than one gunman burst into the room and began firing.
The attackers "just went in the doors and began shooting," a police representative told CNBC, adding there are likely at least "hundreds" of officers at the scene.
Video from the scene showed people lying on the ground, and police helping to support others who were wounded.
A representative from nearby Loma Linda Medical Center told NBC shortly after the attack that the hospital had received four patients and there were at least three more on their way.
California Gov. Jerry Brown had been briefed on the shooting and is closely monitoring the situation, a spokeswoman from his office told CNBC.
"Our thoughts and prayers are with the victims' families and everyone affected by the brutal attack. California will spare no effort in bringing these killers to justice," Brown said in a statement also announcing that he had canceled the annual Christmas tree lighting ceremony planned for Wednesday evening.
The White House said President Barack Obama had also been briefed and asked to be kept updated.
In a previously planned interview, Obama told CBS his team was still monitoring the shooting, but that some policies could be enacted to make such attacks rarer.
"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 could take, not to eliminate every one of these mass shootings, but to improve the odds that they don't happen as frequently," the president said.
Despite reports of a Planned Parenthood center in the area, a representative for the organization told CNBC that the shooting was not at their facility. Last Friday, a gunman stormed a Planned Parenthood clinic in Colorado, killing three people.
During the shooting, the adjacent San Bernardino Public Golf Club was locked down, a representative told CNBC. "Nobody is allowed inside or out at this time," she said. She added that a helicopter is currently staged at the facility but she couldn't provide any other details.
Security at the Inland Center mall in San Bernardino says the entire mall is now closed. Calls to the Macy's store get the recorded message: "We're sorry, due to conditions beyond our control our store is currently closed."
Earlier today, several retailers including Sears started to close after the shooting incident that took place about two miles away. The mall has about 130 stores and is owned and operated by Macerich. CNBC placed a call into Macerich earlier but has not received a call back.
NBC Los Angeles reported public schools in the area had been put on lockdown.
—CNBC's Jane Wells, Reuters and The Associated Press contributed to this report.
Correction: Sgt. Vicki Cervantes is with the San Bernardino Police Department. An earlier wire report misstated her first name.