Washington - President Barack Obama will make a rare primetime address to the nation Sunday laying out how he will keep Americans safe and defeat the Islamic State group, days after 14 people were shot dead in California.
On Saturday, Obama declared that the United States "will not be terrorized," as the IS extremist group praised the couple behind Wednesday's mass shooting in San Bernardino as "soldiers" of its self-proclaimed caliphate.
"We are Americans. We will uphold our values -- a free and open society," Obama said in his weekly radio address.
On Sunday at 8:00 pm from the Oval Office, he will again look to reassure the American people in the wake of the attack, which the FBI is investigating as a possible act of terrorism.
The massacre, if proven to be terror-related, would be the deadliest such assault on American soil since the September 11, 2001 attacks.
The last time Obama made an address to the nation from the Oval Office was in August 2010 to mark the end of US combat operations in Iraq.
Obama will give the latest in the investigation into the California shootings and "the president will also discuss the broader threat of terrorism, including the nature of the threat, how it has evolved, and how we will defeat it," a White House statement said.
"He will reiterate his firm conviction that ISIL (IS) will be destroyed and that the United States must draw upon our values - our unwavering commitment to justice, equality and freedom - to prevail over terrorist groups that use violence to advance a destructive ideology."
Investigators are continuing to comb over evidence and look in the backgrounds of Syed Farook, 28, and his 29-year-old Pakistani wife Tashfeen Malik, the couple who opened fire at a social services center during a holiday party.
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On Saturday, FBI agents raided a home in California belonging to a friend of Farook's, the Los Angeles Times said, citing law enforcement officials.
Top security officials have indicated that the duo had been radicalized but the White House and the FBI say there are no signs that they were part of a larger group or terrorist cell.
However, in a radio broadcast in English, the Islamic State group praised the couple as "soldiers of the caliphate" and martyrs, but did not specifically say they were members of the extremist group.
The heavily armed pair, who wounded 21 others, died in a ferocious shootout with police after a huge manhunt.
The shooting was the worst in the United States in three years and also again revived impassioned debate on gun control, in a country where such mass killings have become routine.
Underlining the handwringing that typically follows such mass killings among some Americans, The New York Times added its voice to the debate, publishing a front-page editorial - the first since 1920 - calling for an end to "the gun epidemic in America."- 'New phase in terror threat' -
Authorities say US-born Farook and Malik, who married last year in Saudi Arabia, where she lived, carefully planned their attack.
David Bowdich, the assistant FBI director in charge of the Los Angeles office, said investigators were examining a Facebook posting in which Malik is believed to have pledged allegiance to IS leader Abu Bakr al-Baghdadi, made around the time of the attack.
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The family's attorneys said that while the couple were devout Muslims, there was no hint they had become radicalized.
Relatives of Farook and his wife have also been at a loss to explain what triggered the killing spree, describing them as a quiet couple who kept to themselves.
Washington has long warned about the threat of homegrown, self-radicalized extremists lurking in its midsts.
"We have moved to an entirely new phase in the global terrorist threat and in our homeland security efforts," Homeland Security Secretary Jeh Johnson told The New York Times.
Terrorists have "in effect outsourced attempts to attack our homeland. We've seen this not just here but in other places," he added.
"This requires a whole new approach, in my view."
Donald Trump, the frontrunner for the Republican presidential nomination, gave little doubt meanwhile as to how he would deal with any terror threat from within.
"I would handle it so tough, you don't want to hear," CNN quoted him saying.