Islamic State said on Saturday that the married couple who killed 14 people in a mass shooting in Southern California were its followers, and FBI agents raided a home apparently belonging to a friend of the husband.
Islamic State's claim came in an online audio broadcast three days after U.S.-born Syed Rizwan Farook, 28, and his Pakistani wife Tashfeen Malik, 29, opened fire with assault rifles on a holiday party for civil servants in San Bernardino, 60 miles (100 km) east of Los Angeles.
The pair, who had left their six-month-old baby daughter with relatives, were killed two hours later in a shootout with police SWAT team members.
Federal agents tore through a garage door to search a house in Riverside, a few miles (km) southwest of San Bernardino, on a street where neighbors said Farook once lived.
An FBI spokeswoman confirmed agents made a "precautionary tactical entry" while serving a federal search warrant, but she declined to give details.
Neighbors named the man who lived there as Enrique Marquez and said he was often seen with Farook.
NBC News said the house belongs to a man who authorities believe bought the rifles used in the attack.
As investigators probe Wednesday's rampage as an act of terrorism, the White House said President Barack Obama will address the nation on Sunday evening to update the public.
If the Dec. 2 mass shooting proves to have been the work of people inspired by Islamist militants, it would be the deadliest such attack in the United States since Sept. 11, 2001.
'Whole new approach'
Jeh Johnson, the secretary of Homeland Security, told the New York Times the country has entered "an entirely new phase" in the global threat from extremists.
Enemies such as Islamic State have "in effect outsourced attempts to attack our homeland," he told the newspaper. "We've seen this not just here but in other places. This requires a whole new approach, in my view."
FBI officials say Farook and Malik seem to have been inspired by foreign militant groups, but that there was no sign they worked with any of them or that Islamic State even knew who they were.
Estranged relatives of Malik say she and her father seemed to have abandoned the family's moderate Islam and become more radicalized during time they spent in Saudi Arabia.
While Obama's team has not yet found evidence the couple was part of an organized group or broader terrorist cell, it said on Saturday that "several pieces" of information "point to the perpetrators being radicalized to violence."
If that turned out to be the case, Obama said in a radio address, it would underscore a long-recognized threat: "the danger of people succumbing to violent extremist ideologies."
Islamic State also claimed responsibility for a Nov. 13 series of attacks in Paris in which gunmen and suicide bombers killed 130 people.
"Two followers of Islamic State attacked several days ago a center in San Bernardino in California," the group's daily online radio broadcast al-Bayan said on Saturday.
An English-language version released later called them "soldiers" of Islamic State, rather than "followers" as in the original Arabic. That inconsistency could not immediately be explained.
The broadcast came a day after Facebook confirmed that comments praising Islamic State were posted around the time of the shooting to an account set up by Malik under an alias.
It was not clear if the comments were posted by Malik, or by someone with access to her page.
Malik moved from Pakistan to Saudi Arabia with her father when she was a toddler, then returned to Pakistan to study pharmacy at a university in Multan from 2007 to 2012.
Estranged relatives interviewed by Reuters in Karor Lal Esan, in Pakistan's central Punjab province, said she and her father appeared to change their views while in Saudi Arabia.
"From what we heard, they lived differently, their mindset is different. ... this is very shocking for us," said school teacher Hifza Bibi, a step-sister of Malik's father.
While at university, Malik was known as a good student and had no known religious extremist tendencies, a local intelligence official said.
Fellow mosque-goers in San Bernadino remember her exuding beauty and happiness two years ago at a reception for hundreds of guests after her marriage to Farook.
The couple had two assault-style rifles, two handguns, 12 pipe bombs and more than 6,000 rounds of ammunition in their home or with them when they were killed, officials said, prompting fears they might have been plotting more attacks.
The mass shooting sparked a new round of the firearms debate with Obama and the New York Times calling for new limits on gun ownership. Many pro-gun voices, including some Republican contenders for the White House, said the new laws would not have stopped the rampage.
The New York Times, in its first front-page editorial for nearly a century, called it "a moral outrage and a national disgrace" that the sort of firearms used in the attack were readily available.