The couple opened fire on a holiday party last Wednesday at a social services agency, killing 14 people.
The investigation into the mass shooting last week in California is now focusing closely on contacts the shooters may have had with radical Islamists in the United States, as opposed to overseas, a government source said on Monday.
The source also said evidence in hand so far suggests the radicalization of the shooters, Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, started no earlier than 2014.
The couple opened fire on a holiday party last Wednesday at a social services agency in San Bernardino, California, killing 14 people and wounding 17 others before fleeing, authorities said.
The shooting rampage at the Inland Regional Center about 60 miles (100 km) east of Los Angeles marked the deadliest U.S. gun violence since the massacre at Sandy Hook Elementary School in Newtown, Connecticut, in December 2012, in which 27 people, including the gunman, were killed.