US President Barack Obama has vowed the United States "will not be terrorised" as the Islamic State (IS) group praised the couple behind the California mass shooting as "soldiers" of its caliphate.
"It is entirely possible that these two attackers were radicalised to commit this act of terror," Obama said of Syed Farook, 28, and his 29-year-old Pakistani wife Tashfeen Malik, who slaughtered 14 people and wounded 21 others at a year-end office party in San Bernardino on Wednesday (local time).
The pair later died in a shootout with police.
The White House said in a statement that Obama had met with top security officials who told him that while there was evidence Farook and his wife had been radicalised, nothing at this point indicated that they were "part of an organised group or formed part of a broader terrorist cell."
In a radio broadcast in English on Saturday, the Islamic State group praised the couple as "soldiers of the caliphate" and martyrs but did not specifically say they were members of the terror group.
The mass shooting, the worst in the United States in three years, has again revived the debate on gun control in a country where such killings have become routine.
"Right now, people on the No-Fly list can walk into a store and buy a gun. That is insane," said Obama, who has repeatedly urged tougher controls.
The New York Times added its voice to the debate on Saturday publishing a front-page editorial - the first since 1920 - calling for an end to "the gun epidemic in America."
"It is a moral outrage and a national disgrace that civilians can legally purchase weapons designed specifically to kill people with brutal speed and efficiency," it said.
Authorities on Saturday said that while the two rifles used by the couple in the massacre had been purchased legally, they had been altered to make them more powerful and were illegal under US law.
"One of the rifles was modified to accept large capacity magazines and the receiver on the other had been altered to fire as a machine gun but it did not function," Meredith Davis, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, told AFP.
Authorities believe the couple, who married last year in Saudi Arabia, where Malik lived, carefully planned their attack which is being investigated as an act of terrorism.
"We have uncovered evidence that has led us to learn of extensive planning," David Bowdich, the assistant FBI director in charge of the Los Angeles office, told reporters.
Relatives of Farook and his wife - who had a six-month-old daughter - have been at a loss to explain what triggered the killing spree.
"I can never imagine my brother or my sister-in-law doing something like this. Especially because they were happily married, they had a beautiful six-month-old daughter," Farook's sister Saira Khan told CBS News.
A US defence official, meanwhile, confirmed that Farook's brother was a decorated Navy veteran who won medals for his service during America's "war on terror."