San Bernardino County employees have begun returning to work under tighter security, five days after a co-worker and his wife killed 14 of their colleagues at a holiday party.

Crisis counselling teams were on Monday reaching out to families of those killed and wounded on December 2 in the fusillade from US-born Syed Rizwan Farook, 28, and his Pakistani wife, Tashfeen Malik, 29. The couple died hours later in a shootout with police.

Most San Bernardino County workers returned to work on Monday for the first time since the massacre that President Barack Obama has called an act of terrorism, county officials said at a news conference.

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Staff from the county’s environmental health department, where Farook worked as an inspector, were expected to return to work on December 14.

San Bernardino County Supervisor Janice Rutherford said the best way to honour the shooting victims was to return to normal operations.

“We can’t be afraid,” she said at the news conference.

Under the tightened security protocols at county buildings, some security guards who had previously been allowed only to observe and report incidents are now authorised to carry firearms and “engage and detain” assailants, James Ramos, chairman of the San Bernardino County Board of Supervisors, said.

The Federal Bureau of Investigation continued to seek a motive for the attack by Farook and Malik. Agents believe the couple had been planning more violence because of their cache of ammunition and pipe bombs.

Investigators also are trying to determine if Malik was responsible for taking the couple’s Muslim faith to the point of radicalisation.