The suspected San Bernardino attackers left behind 12 pipe bombs and more than 2,000 rounds of ammunition at their home, police said Thursday.
Wednesday's shooting that killed 14 people and wounded 21 at social services facility The Inland Regional Center has been called the nation's deadliest gun massacre since Sandy Hook.
The guns used by the couple suspected of the massacre were purchased legally in the United States, according to federal officials. The attackers had purchased their two handguns, but their two assault-style rifles were purchased by someone who is now under investigation, San Bernardino police chief Jarrod Burguan told reporters on Thursday. The shooters wore black tactical gear, but did not have ballistic vests, he added.
NBC News learned Thursday that the attackers left behind a remote-controlled explosive device at the scene of the shooting as they left in their black SUV with the controller to the bomb in their hands.
Multiple federal and regional sources familiar with the investigation told NBC News that the controller was similar to the model car controller used in the Boston Marathon bombing. This would indicate that the attackers appear to have calculated a heavy law enforcement and public safety response and left behind a device that could have caused even greater causalities. Burguan said the remote appeared not to have worked for them.
California governor Jerry Brown has since called for increased caution.
At least 75-80 people were in the room when the shooting occurred, Burguan said, adding that the attack took place at the Center's department for public health. The majority of the victims were county employees, he noted.
While the couple appeared to have planned the attack, the motive has not been determined, investigators said. At least 10 of the 21 wounded were in critical condition early Thursday, NBC reported. Authorities will start releasing names of victims "fairly soon," Burguan said.
Police named the suspects as Syed Rizwan Farook, 28, a U.S. citizen who had worked for San Bernardino County for at least five years, and Tashfeen Malik, 27, whose nationality has not been determined. Both were killed in a police shootout following Wednesday morning's attack.
NBC said the couple left their 6-month-old daughter with her grandmother before they carried out the attack at the Inland Regional Center, a complex of buildings that serves people with developmental disabilities.
The FBI's David Bowdich said Thursday that Malik traveled to the U.S. with a Pakistani passport, and that Farook had traveled to Pakistan. Intelligence sources told NBC News that Farook appears to have been in contact with overseas individuals who are persons of interest to U.S. authorities. Authorities said he had also been in touch with persons of interest in the Los Angeles area who have expressed Jihadist-oriented views.
Investigators are still looking into whether the massacre in San Bernardino was an act of terrorism, President Barack Obama said Thursday from the Oval Office.
"We don't know at this point the extent of their plans, we do not know their motivations," Obama said.
The president said he spoke with San Bernardino's mayor, and thanked him for managing the situation with calm and clarity, but he emphasized that officials do not yet know why the attack occurred.
Obama renewed his call for the country to take "basic steps" to make it harder for certain individuals to get access to weapons.
"We see the prevalence of these kinds of mass shooting in this country, and I think so many Americans sometimes feel as if there is nothing we can do," Obama said. "We can't just leave it to our professionals to deal with the problem of these kinds of horrible killings — we all have a part to play. And I do think that as the investigation moves forward, it's going to be important for all of us — including the legislatures — to see what we can do to make sure that when individuals decide that they want to do somebody harm, we're making it a little harder for them to do it because right now it's just too easy."
Meredith Davis of the Bureau of Alcohol, Tobacco, Firearms and Explosives said investigators are now working to make a connection to the last legal purchaser and the rampage, according to the AP. She said all four guns were bought four years ago but she did not say whether they were purchased out of state or how and when they got into the hands of the two shooters.
The mass shooting in San Bernardino, 60 miles east of Los Angeles, came three years after a gunman killed 26 people at the Sandy Hook Elementary School in Newtown, Connecticut, before killing himself.
The San Bernardino couple had been at a party held for Farook's fellow employees. Farook, a county health inspector, became involved in some kind of dispute and left the premises "angry," Burguan said.
"These people came prepared to do what they did as if they were on a mission," Burguan said. "They were armed with long guns, not with handguns."
Farook, who police said was born in the U.S. in 1987, and Malik later returned to the center armed with multiple weapons, including .223-caliber assault rifles and handguns, and wearing combat-style clothing, and opened fire on the gathering before fleeing in a black SUV.
The police chief said authorities had not ruled out terrorism or a workplace dispute as the motive. Multiple local, state and federal agencies are investigating.
"It's all hands on deck for the FBI evidence response teams and the other investigators," former FBI Assistant Director Chris Swecker said Thursday on CNBC's "Squawk Box." "They'll be looking hard at phones, they'll be looking at social media, they'll be looking at computers. They'll be dissecting this person's life and social network."
U.S. Attorney General Loretta Lynch addressed the shooting in a Thursday morning address, calling it an "unspeakable crime."
"Whatever the result of the investigation — we don't know a lot right now. But one thing is clear, that violence like this has no place in this country," she said. "This is not what we stand for, this is not what we do, this is not what we work for, it's not what we live for. It's antithetical to our values."
People at the party at the Inland Regional Center said there was no warning of the shooting. Anies Kondoker, an environmental engineer who works for the county, was inside the conference room when gunfire erupted at around 11 a.m. PT. She was shot twice in the hand and once in the stomach but doctors said no vital organs were hit, her husband, Salahin, told NBC.
"They just started shooting ... they didn't yell or say anything beforehand," her husband told NBC News.
Burguan told reporters that the shooters' actions indicated "some element of planning."
Read More Calif. shooting has 'no place' in US: AG
"I don't think they just ran home and put on some tactical-type clothes" and came back on the spur of the moment, he said.
Burguan confirmed that Farook and Malik were the man and woman who were killed in a shootout with police about five hours after the San Bernardino attack.
Police had earlier said as many as three attackers were involved, but Burguan said he was "reasonably confident at this point that we have two shooters and we have two suspects who are deceased."
Reports first surfaced around 11:15 a.m. PT of a shooting at the center. Police were at the scene within four minutes, Burguan said.
The hunt for the black SUV led police to an address at the neighboring town of Redlands, and police chased a vehicle that was seen leaving that address. Burguan said the chase ended with officers engaging in a gun battle with the two suspects, both of whom were armed with assault rifles and handguns. A police officer was injured but not seriously, Burguan said.
The attack came less than three weeks after 132 people were killed in multiple assaults in Paris. The so-called Islamic State claimed responsibility for the Nov. 13 attacks, heightening fears that other Western targets were at risk from the terrorist group.
Even before police had formally identified Farook and Malik, a news conference was called by the Los Angeles chapter of the Muslim advocacy group Council on American-Islamic Relations, where a man who said he was Farook's brother-in-law told reporters he was bewildered by the San Bernardino attack.
Farhan Khan, who Reuters reported is married to Farook's sister, said: "Why would he do that? Why would he do something like this? I have absolutely no idea, I am in shock myself."
Hussam Ayloush, executive director of CAIR in the Los Angeles area, appealed to the public not to jump to conclusions about the suspects' motives.
"Is it work? Is it rage-related? Is it mental illness? Is it extreme ideology?" he said. "We just don't know."
The killings also come less than a week after a gunman killed three people and wounded nine at a shooting at a Planned Parenthood Clinic in Colorado. There have been more than 350 U.S. shootings in which four or more people were wounded so far this year, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of U.S. gun violence.
— NBC News, the AP and Reuters contributed to this report.