A week-old investigation into the shooting rampage that left 14 people dead at a holiday party in southern California turned on Thursday to a small lake near the scene of the massacre, where divers searched for evidence.

The FBI also revealed that investigators were looking into any connections there may have been between one of the two killers in San Bernardino last week and four men arrested in 2012 in a separate federal terrorism case brought in nearby Riverside, California.

Thursday's underwater search stemmed from unspecified leads indicating the married couple who carried out the shooting spree had been in the vicinity of Seccombe Lake in San Bernardino on the day of the killings, said David Bowdich, assistant director of the FBI field office in Los Angeles.