Authorities also have evidence that Syed Rizwan Farook, 28, and his spouse Tashfeen Malik, 29, had engaged in firearms target practice near their Southern California home within days of last week's deadly shooting rampage, according to the Federal Bureau of Investigation.

The latest disclosures in the FBI-led investigation came as San Bernardino County employees began returning to work under tighter security, five days after Farook, an environmental health inspector for the county, and Malik opened fire with assault-style rifles on a holiday gathering of his colleagues.

The FBI said last week that authorities are investigating the mass shooting as an 'act of terrorism', noting that Malik, a Pakistani native who lived most of her life in Saudi Arabia, was believed to have pledged allegiance on Facebook to the leader of the militant group Islamic State.

A day after Democratic President Barack Obama urged Americans in a televised White House address to avoid scape-goating of Islam as a religion, Republican presidential front-runner Donald Trump called for a ban on Muslims entering the United States.

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