The FBI says it is investigating the gun massacre in San Bernardino as an "act of terrorism", with the wife having pledged allegiance to a leader of Islamic State (IS).
According to the Los Angeles Times, the husband had contact with people from at least two militant organisations overseas, including the al Qaeda-affiliated Nusra Front in Syria.
Both US-born Syed Farook and his wife, Tashfeen Malik, died in a shootout with police hours after Wednesday's attack on a holiday party at a social services centre.
"Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism," David Bowdich, assistant director of the FBI's Los Angeles office, said at a news conference.
He pointed, in particular, to the discovery of two crushed cell phones left by the couple in a trash can near their rented townhouse.
Mr Bowdich said it was hoped data retrieved from the phones and other electronic devices seized in the investigation would lead to a motive for the attack.
The couple had two assault-style rifles, two semi-automatic handguns, 6,100 rounds of ammunition and 12 pipe bombs in their home or with them when they were killed.
Mr Bowdich said they may have been planning another attack.
He said neither Farook, 28, or Malik, was on the intelligence radar, even though they had "telephonic connections" with the subjects of FBI investigations.
FBI Director James Comey said on Friday there is no current indication the couple were part of a larger organised network, group or cell.
Aamaq, a news agency affiliated with IS, has said the pair were "supporters" of the extremist group, although it stopped short of claiming responsibility for the attack.
It has also emerged Malik, a native of Pakistan, praised IS on Facebook as she and her husband began spraying bullets at his colleagues.
According to law enforcement officials, the 29-year-old swore allegiance to IS leader Abu Bakr al Baghdadi.
A Facebook executive says Malik posted the material under an alias account at 11am on Wednesday.
That was about the time she and her husband stormed into the social services centre and opened fire with assault weapons, killing 14 people and injuring 21.
An attorney for the Farook family told reporters on Friday that his relatives said he had been teased about his beard by colleagues.
Malik, who grew up in Saudi Arabia, came to the US last year under a fiancee visa, sponsored by her husband.
Sky News has been allowed with other media to film inside the couple's suburban home in Redlands, a neighbouring city to San Bernardino.
A child's book of the Quran, family pictures and shredded documents were among items in the house, where the killers lived with their six-month-old daughter.
US President Barack Obama spoke on Friday with UK Prime Minister David Cameron about the fight against IS, a day after British bombers made their first strikes on the group in Syria .