WASHINGTON - Director of US Federal Bureau of Investigation James Comey said Wednesday both shooters of San Bernardino carnage were radicalized long before they started online dating.

Tashfeen Malik, (L), and Syed Farook are pictured passing through Chicago's O'Hare International Airport in this July 27, 2014 handout photo obtained by Reuters December 8, 2015.  [Photo/Agencies]

"They were actually radicalized before they started ... dating each other online, and as early as the end of 2013, they were talking to each other about jihad and martyrdom before they became engaged," said Comey here at a congressional hearing.

The revelation appeared to contradict an early theory which claimed that Tashfeen Malik, 27, the female shooting suspect who was born in Pakistan, may have converted her husband Syed Farook, a 28-year-old US citizen, to an extremist ideology.

Comey said investigators so far had not determined whether the shooters' start of communications was arranged by any group or developed naturally on its own.

"It would be a very, very important thing to know," Comey told US lawmakers.

According to U.S. investigators, Malik moved from Pakistan to the United States in July, 2014 and married Farook the following month. Farook was born in Chicago, Illinois and raised in southern California.

Echoing senior officials from the US homeland security and intelligence community, Comey said Wednesday the incident represented new challenges to homeland security, in which violent extremists appeared to prepare for and launch attacks on their own.

The married suspects gunned down 14 people and injured 21 more at a holiday party held at a southern California social services center on Dec. 2. According to local police, "some degree of planning" was involved in the horrendous shooting spree.

The couple had 1,600 rifle and 9-millimeter rounds of ammunition when they were killed in a shootout with police, together with another 2,000 9-millimeter rounds and 2,500 .223 rifle rounds at home, said the police, adding that 12 pipe bombs and tools to make bombs were also found at their home. Those did not include the hundreds of rounds they fired in the shooting and gunfire with the police on the scene.

U.S. President Barack Obama called the assault "an act of terrorism" in a prime time speech Sunday, but stressed that no evidence had so far pointed to the shooters' affiliation with any terrorist groups, including the Islamic State.