Three people linked to the couple responsible for December's mass shooting in San Bernardino, California are arrested on conspiracy, marriage fraud and other charges unrelated to the massacre, US prosecutors said.

Apple boss Tim Cook has hit back at the FBI over the handling of a court order to help unlock the iPhone of San Bernardino killer Syed Rizwan Farook.

The San Bernardino attackers began their relationship online and then met at the 2013 Hajj pilgrimage, according to a visa application.

As anti-Muslim sentiment swells following the December 2 massacre in San Bernardino, California, by a young Muslim couple inspired by Islamic State, many Muslim parents and their children say they fear for their safety and are struggling with their American and Muslim identities.

U.S. authorities in Pakistan could have sought - but did not - a full background security investigation of San Bernardino shooter Tashfeen Malik before granting her a visa to enter the United States in 2014, according to a U.S. official familiar with the matter.

Donald Trump's anti-Muslim rhetoric undermines US national security by boosting the Islamic State (IS) group, the Pentagon warns.

President Barack Obama says the US "will not be terrorised" by Wednesday's mass shootings in San Bernardino, California.

Lawyers representing the family of the two San Bernardino attackers say, relatives are "in complete shock" over the shooting.

A US-born Muslim who along with his wife gunned down 14 people in California may have been radicalized and had been in contact with known terrorism suspects, news reports say.