US authorities could soon be snooping on social media posts from would-be immigrants in a new effort to close security gaps in the country’s visa system.

US authorities could soon be snooping on social media posts from would-be immigrants in a new effort to close security gaps in the country's visa system.

A couple who massacred 14 people at a holiday party in California were discussing martyrdom online a year before they met in person and married, FBI director James Comey said.

The Pakistani woman who joined her US-born husband in killing 14 people in a commando-style assault on his co-workers in California is at the centre of a massive FBI terrorism investigation, yet she remains shrouded in mystery.

Investigators in the US believe the woman who carried out Wednesday's gun attack in California with her husband had sworn allegiance to the so-called Islamic State.

There have been more than 350 shootings this year, in which four or more people were wounded or killed in the US, according to the crowd-sourced website shootingtracker.com, which keeps a running tally of gun violence.