NEW YORK: There is no evidence the married couple who killed 14 people in California this month were part of a terrorist cell, the head of the FBI said on Wednesday, echoing investigators' views that the pair were inspired by, rather than organised by Islamic State.

The militant group has “revolutionised” terrorism by seeking to inspire such small-scale attacks, FBI Director James Comey said, noting the group uses social media, encrypted communications and slickly produced propaganda to recruit followers around the world.

“Your parents' al Qaeda was a very different model than the threat we face today,” Comey told a counterterrorism conference in New York.

However, he said that while the perpetrators of the December 2 shootings in San Bernardino, California — Syed Rizwan Farook, 28, and Tashfeen Malik, 29 — had expressed support for “jihad and martyrdom” in private communications, they never did so on social media.

Comey said the Federal Bureau of Investigation currently has “hundreds” of investigations in all 50 US states involving potential Islamic State-inspired plots.

His remarks came as Americans are jittery two weeks after the San Bernardino attack.

Comey said the group has perfected the use of social media, and Twitter in particular, to contact potential followers in the United States and elsewhere.

“Twitter works as a way to sell books, as a way to promote movies, and it works as a way to crowd-source terrorism — to sell murder,” Comey said.

Islamic State also employs “end-to-end” encryption when communicating with individuals who it believes are willing to carry out killings in its name, Comey said.

That has posed a significant challenge for investigators, who often find themselves stymied even when they have court orders giving them access to devices.

But Comey said he is convinced that law enforcement and technology companies can work together to solve that problem without compromising personal privacy.

“We are not going to break the Internet,” he said. “We are not going to jeopardise people's security.”

Islamic State is based in Iraq and Syria, where it controls a large area of territory as it seeks to carve out a caliphate. It claimed responsibility for attacks in Paris on November 13 that killed 130 people.