FBI divers picked through the bottom of a San Bernardino lake on Saturday for a third day, seeking evidence related to a married couple who massacred 14 people at a holiday party, in what the bureau has called an act of terror inspired by Islamic State.

Federal Bureau of Investigation spokeswoman Laura Eimiller said in an email divers retrieved objects from Seccombe Lake, as they did the previous day. But she declined to say what those were or whether they appeared to be tied to the mass shooting.

The San Bernardino lake, nestled in a public park, about 2.5 miles (4 km) north of the site of the shooting, is believed to be littered with junk and debris.

US officials have said their investigation has yet to turn up evidence that foreign militants directed Syed Rizwan Farook, 28, or Tashfeen Malik, 29, when the married pair stormed a holiday gathering of his co-workers at a regional center in San Bernardino on Dec. 2 and opened fire with assault rifles.

The couple shot dead 14 people and wounded more than 20 in a rampage the FBI said it is treating as an act of terrorism inspired by Islamic militants. If that is confirmed, it would mark the most lethal such attack on U.S. soil since Sept. 11, 2001.

Farook, the US-born son of Pakistani immigrants, and Malik, a Pakistani native he married last year in Saudi Arabia, were killed in a shootout with police hours after their assault in San Bernardino, 60 miles (100 km) east of Los Angeles.

CNN and other media have reported the FBI divers at the lake were looking for a computer hard drive that belonged to the couple, but Eimiller declined to confirm that.

The underwater search stemmed from leads indicating Farook and Malik had been in the vicinity of Seccombe Lake on the day of the killings, the FBI has said.

The FBI has determined that the couple declared they were acting on behalf of the Islamic State. But FBI Director James Comey has said there was no evidence the militant group controlling vast swaths of Iraq and Syria was even aware of them prior to their attack.

On Friday, a fire that appeared to have been intentionally set burned the entrance to a mosque in Southern California's Coachella Valley, some 75 miles (121 km) from San Bernardino, raising concerns of a possible reaction to the shooting.

A 23-year-old man was arrested on suspicion of arson and for committing a hate crime, according to the Riverside County Sheriff's Department, which has not said if he was motivated by the shooting in targeting the mosque.

Muslim Americans across the country have said they are worried about a backlash, as happened in the aftermath of the Sept. 11, 2001, attacks. – Reuters