The young California parents who killed 14 people in a workplace rampage last week had both been “radicalized” into following an extreme form of Islam, an FBI official said Monday.

“We have learned and believe that both subjects were radicalized and had been for quite some time,” David Bowdich, the FBI’s assistant director in charge of the Los Angeles office, told reporters.

In the five days since Syed Farook and Tafsheen Malik opened fire on a holiday luncheon attended by San Bernardino County health workers, authorities have been trying to determine why the couple, who seemed outwardly quiet and withdrawn, acted with such sudden violence, leaving behind a 6-month-old daughter.

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The questions have focused primarily on Malik, 29, a Pakistani national who reportedly posted a Facebook message just before the attack praising ISIS.

But Bowdich acknowledged that investigators still do not know how, or where, the couple were influenced to act on their radicalization. It’s not even clear if any one else is responsible, since it is possible to be self-radicalized through the internet, Bowdich said.

This story originally appeared on NBCNews.com