U.S. President Barack Obama has vowed federal investigators will uncover what motivated a married couple in California to shoot and kill 14 people.
"We are strong. And we are resilient. And we will not be terrorized," Obama said in his weekly radio address on December 5.
The White House announced Obama will make a rare televised address to the nation on December 6 to discuss the steps the government is taking to safeguard against terrorism. It will be just the third such speech he has made since becoming president nearly seven years ago.
Tashfeen Malik and Syed Rizwan Farook were killed in a shoot-out with police after the December 2 attack during a holiday party at a social-services agency in San Bernardino, California.
The Federal Bureau of Investigation (FBI) is investigating the massacre as an "act of terrorism" after Malik was believed to have pledged allegiance to a leader of the militant group Islamic State (IS).
"It is entirely possible that these two attackers were radicalized to commit this act of terror," Obama said. "And if so, it would underscore a threat we've been focused on for years -- the danger of people succumbing to violent extremist ideologies."
An FBI spokeswoman said on December 5 that agents had raided a house on the street where Farook once lived. She did not say what the agents were looking for.
Earlier on December 4, FBI Director James Comey told reporters in Washington that the probe had become a federal terrorism inquiry.
"The reason for that is that the investigation so far has developed indications of radicalization by the killers and of potential inspiration by foreign terrorist organizations," he said.
Comey said there were no indications that the couple was part of a terrorist cell or a network of terrorism.
He said neither was under investigation by the FBI and that even though one or both had had contacts with other individuals who were being investigated by the FBI, none was significant enough that they would have put the couple on the FBI's "radar screen."
However, the IS group said in an online radio broadcast on December 5 that two followers of the extremist group had carried out the attack.
"Two followers of Islamic State attacked several days ago a center in San Bernardino in California," the group's daily Al-Bayan broadcast said.
Earlier, David Bowditch, assistant director for the FBI's Los Angeles office, said investigators were looking at the "digital fingerprints" left on computers and elsewhere and that two partially destroyed cell phones had been recovered from trash cans not far from the scene of the shooting in San Bernardino, about 100 kilometers east of Los Angeles.
Officials say Malik and Farook used automatic weapons, handguns, and pipe bombs in the attack. Farook had attended the holiday party at the agency and then reportedly left in an agitated state before returning with his wife, heavily armed and wearing tactical vests.
Authorities have said that the couple fired as many as 75 rounds into the room where the holiday party was being held before fleeing and that they had more than 1,600 rounds of ammunition left when they were killed.
Police later found 12 pipe bombs, tools to make more explosives, and well over 4,500 rounds of ammunition at the townhouse they were renting.
The couple was killed hours later after a car chase and a fierce gunbattle with police.
Malik was a Pakistani native who moved to the United States in 2014 on a fiancee visa. Farook, who was born in Chicago to Pakistani parents and raised in southern California, was a 28-year-old health inspector for the county.
Malik had been living in Saudi Arabia before marrying Farook two years ago.
Officials have said that while the killings may have been inspired by the IS group, there was no evidence that it had directed the attack.
Meanwhile, other law-enforcement officials were quoted by news agencies and CNN as saying that Malik had pledged allegiance to the IS group and its leader on Facebook.
The AP quoted an unnamed official as saying Malik made her posts under an alias and deleted them before she and Farook went on their rampage.
Another U.S. official was quoted as saying that Malik expressed "admiration" for the extremist group's leader on Facebook under the alias account.
But the official said there was no sign that anyone affiliated with IS communicated back with her, and there was no evidence of any operational instructions being conveyed to her.
Bowditch said neither Farook nor Malik was under prior investigation and he could not confirm the Facebook posting, saying investigators were still looking at it.
Saudi officials also said that Malik had not raised any red flags while she was living in Saudi Arabia.
The shooting has sparked heated debate in the United States about the wide availability of guns and the prevalence of mass shootings.
In a rare move, The New York Times used its front page on December 5 to call for stronger gun laws in the United States, and specifically called for outlawing the kind of rifles used in the California attack.
The attack also stoked fears that people who are radicalized or supportive of IS could enter the United States alongside refugees from Syria or elsewhere.
Americans were already nervous about terrorism after IS militants killed 130 people in Paris on November 13. The United States is leading an international coalition fighting the group in Syria and Iraq.
The shooting was the deadliest in the United States since 2012.
A recently announced plan by the Obama administration to bring thousands more refugees from Syria's civil war to the United States has been harshly criticized by many lawmakers and Republican presidential candidates.
IS leaders, whose fighters have seized swaths of territory in Iraq and Syria as part of an effort to create a theocratic state called a caliphate, have urged sympathizers worldwide to commit violence in their countries.