Investigators were drawn to Seccombe Lake in San Bernardino, California, by unspecified leads indicating the married couple who carried out the shooting had been in the vicinity the day of the killings, said David Bowdich, assistant director of the FBI field office in Los Angeles.

Seccombe Lake Park lies about 2.5 miles (4 km) north of the Inland Regional Center, the social services agency where 14 people were killed and 22 others injured when Syed Rizwan Farook and his wife, Tashfeen Malik, stormed a holiday gathering of his co-workers there on Dec 2 and opened fire with assault rifles.

Farook, 28, a US born son of Pakistani immigrants, and Malik, 29, a Pakistani native he married in Saudi Arabia last year, were slain hours later in a shootout with police.

The couple's motives remain unclear. But if the crime proves to have been the work of killers driven by militant Islamic ideology, as the FBI suspects, it would mark the deadliest such attack on US soil since Sept 11, 2011.

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