Federal investigators in California have appealed for the public's help to piece together the whereabouts of the San Bernardino shooters during a brief period after the massacre.

David Bowdich, head of the FBI office in Los Angeles, said that while investigators had largely determined the movements of Syed Farook and his wife Tashfeen Malik on the day of the massacre, 2 December 2015, there were 18 minutes unaccounted for.

14 people were killed and 22 were injured in the shooting at a county office building.

He said investigators have been unable to determine where the couple was between 12.59 pm and 1.17 pm.

The period falls between the time of the shooting, which took place at 10.59 am, and the time the couple was spotted by police and died in a firefight at around 4pm.

"So why is that 18 minutes so important?" Mr Bowdich told a press conference. "It's important because we want to ensure that we know whether or not they stopped at any locations, any residences, any businesses that we don't already know about."

He said it was possible that the couple may have stopped at a storage facility or had contact with someone during those 18 minutes.

"We just don't know," Mr Bowdich said.

"It may not be an important fact, but until we close that gap we don't know."

He said investigators had determined that after the attack the couple drove around the San Bernardino area, where they lived, stopping at several locations, including a lake.

Mr Bowdich said a search of the lake had produced nothing linked to the investigation.

He also added that while the assault, the deadliest terror attack in the United States since September 11, was apparently inspired by foreign terrorist networks, there was no indication of a direct link to a specific group.