Authorities were still trying Thursday to establish a motive for the deadliest U.S. mass shooting in nearly three years, even as they revealed that the two attackers had amassed a large stockpile of explosives and ammunition.

The rampage killed 14 people, wounded 21 and locked down a swath of Southern California for much of the day Wednesday as investigators scrambled to determine whether they were looking at a terrorist attack or an extremely unusual and lethal case of workplace violence.

The killers were a young husband and wife who welcomed the birth of a daughter just six months ago and showed no outward sign of Islamist radicalization, psychological distress or a desire for mayhem. The couple were slain in a wild police shootout on a residential street four hours after the massacre.

The FBI, which has authority to investigate potential terrorism, announced Thursday that it had taken over the investigation. Authorities were carefully picking through three crime scenes: the Inland Regional Center, where the mass shooting occurred; the San Bernardino street where the couple died in the gun battle with police; and the couple's rented home in Redlands, Calif., where robots helped investigators root out an arsenal of pipe bombs and thousands of bullets.

Police identified the shooters as Syed Rizwan Farook, 28, a county health worker born in Chicago, and Tashfeen Malik, 27, his Pakistani wife, who was in the United States on a visa.

Farook, who had a college degree in environmental health and a steady job as a health inspector, traveled to Saudi Arabia and Pakistan last year and returned with Malik, whom he had met online. They were married in the United States, police said.

Authorities have said the two were not on any watch lists. A senior U.S. law enforcement official said that Farook was in contact with persons of interest with possible ties to terrorism but that these were not "substantial" contacts.

Farook's supervisor, Amanda Adair, who also went to college with him at California State University at San Bernardino, said he "got along with everybody, but he kept his distance." She said that she "can't imagine [the shooting] was about work" and that she had no inkling that Farook had the capacity for such violence.

Without a firmly established motive, authorities said Thursday that they could not determine whether they were dealing with terrorists, a disgruntled worker who had enlisted his wife in his cause, or some kind of hybrid of those two scenarios.

"We do not yet know the motive," David Bowdich, assistant director in charge of the FBI's Los Angeles office, said at a news conference. "It would be irresponsible and premature for me to call this terrorism."

The case doesn't fit any familiar template. If it was terrorism, why would the shooters target co-workers in a small city that many Americans couldn't find on a map, rather than some more spectacular target? If it was workplace violence, why build up an arsenal of bullets and pipe bombs?

"It is possible this was terrorist-related, but we don't know," President Obama said Thursday in somber remarks in the Oval Office. "It is also possible this was workplace-related."

Mark Pitcavage, director of the Center on Extremism for the Anti-Defamation League, said that "based on what is known now about the case, it certainly is unusual and does not fit neatly into any of the traditional models of violence that we're familiar with."

Police said Farook and Malik were dressed in tactical gear and armed with rifles, handguns and multiple ammunition magazines when, at about 11 a.m., they strode into a conference room where about 80 people were gathered for a staff training session that was transitioning into a holiday party.

They opened fire, spraying 65 to 75 rounds and hitting more than a third of the people. A bullet struck a sprinkler head, and the sprinklers began soaking the room as the fire alarms went off. The shooters fled in a rented black Ford Expedition, leaving behind a bag with three pipe bombs designed to be triggered with a remote-control device from the SUV. The device malfunctioned.

San Bernardino police Lt. Mike Madden, the first law enforcement officer to arrive at the center, described the fresh scent of gunpowder and a horrifying scene for which years of training had not fully prepared him.

"The situation was surreal," Madden said Thursday. "It was unspeakable, the carnage we were seeing."

Farook had been with his colleagues at the party earlier in the morning, police said. Authorities could not say conclusively whether there had been a dispute that led Farook to leave the party. But police said a survivor of the shooting told them that Farook slipped away before the massacre.

That tip led police to check Farook's name, which led to the discovery that he had rented an SUV that matched the description of the getaway car.

Soon, authorities were staking out the couple's home in Redlands, a suburb 15 minutes to the east. Several hours after the shooting, the SUV rolled by and then sped away, and police gave chase.

The SUV stopped on San Bernardino Avenue, a few miles from the massacre. Cellphone videos captured the furious gun battle that followed. Police said the couple fired 76 rifle rounds; police fired 380.

Farook and Malik died. Two officers were injured, but the wounds were not life-threatening. The SUV, so riddled with bullets that it looked as if it had been hit with a bomb, was due back at the rental agency that day, police said.

Police found more than 1,600 rounds of ammunition on or near the couple, suggesting that they were prepared for a long siege. Police recovered two assault rifles and two 9mm pistols, all legally purchased, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives. Two of the weapons were traced to one of the assailants, said Dannette Seward, an ATF spokeswoman, while the other two were traced to another person who has not been publicly identified.

"The FBI is chasing down any contacts these two may have had and whether those contacts are indicative of radicalization or external plotting or are purely incidental," said Rep. Adam B. Schiff (California), the ranking Democrat on the House Intelligence Committee.

Schiff, who was briefed Thursday on the attack, said that "on the basis of what I heard and where the [FBI] was, I wouldn't conclude that there was radicalization here."

The congressman said the shooting also did not appear to be "an act of spontaneous workplace violence." But, he said, it could have been the culmination of a longer-term grievance.

"There appears to be a degree of planning that went into this," San Bernardino Police Chief Jarrod Burguan said. "Nobody just gets upset at a party, goes home and puts together that kind of elaborate scheme or plan."

At a morning news conference, authorities said they had gathered a number of items that were being analyzed to investigate the couple's digital trail, including thumb drives, computers and cellphones. But the two had left behind remarkably little in the way of a digital record - no apparent criminal record, no Facebook page, no Twitter account.

Speaking to the Los Angeles Times, co-workers who knew Farook described him as a quiet, polite man who held no obvious grudges against people in the office. The office had recently held a shower for the couple's baby, and the two seemed to be "living the American Dream," Patrick Baccari, a fellow inspector who shared a cubicle with Farook, told the Times.

A number of families in this city were shattered by Wednesday's violence. On Thursday, officials released the names of the 14 people slain at the holiday party. The eight men and six women ranged in age from 26 to 60. One ran the coffee shop in the building. Twelve of the 14 were county employees.

Shaken, too, were Muslims in Southern California. At the Islamic Society of Corona-Norco, Ray Abboud said Muslims were horrified by the shooting. He said he fears people will paint Muslims with one brush.

"It breaks our hearts to see 14 people die," Abboud said. "We feel sorry for everything that happened, but we can't blame ourselves for being Muslim."

He said people in the community were keeping a close watch on their children "to make sure they don't fall into any crazy stuff."

Before the attack, Farook and Malik dropped off their 6-month-old daughter with Farook's mother, saying they had a doctor's appointment, according to Hussam Ayloush, executive director of the Council on American-Islamic Relations in Los Angeles. The council organized a news conference late Wednesday featuring Farhan Khan, who is married to Farook's sister.

"I have no idea why he would do something like this," Khan said of his brother-in-law. "I cannot express how sad I am today."