Divers from the FBI searched a lake in San Bernardino, California, on Thursday for a computer hard drive missing from the home of the suspects involved in last week’s deadly shooting at a social service center in the city, an FBI spokesman reportedly said. The news comes as authorities seek to determine the motive behind the attack.

The search at Seccombe Lake Park, about 2 miles from the assault site, could take days, David Bowdich, assistant director in charge of FBI’s Los Angeles office, told reporters. Authorities believe the suspects -- Syed Rizwan Farook and his wife, Tashfeen Malik -- visited the park on the day of the shooting.

“We would be remiss not to go into this lake and conduct a thorough search,” Bowdich said, according to the Wall Street Journal, vowing that the investigation would leave “no stone unturned.”

Farook and Malik opened fire at the Inland Regional Center in San Bernardino on Dec. 2, killing 14 people and injuring 20 others. The couple, who officials say became radicalized some years ago, was killed in a standoff with police hours after the attack.

Bowdich also reportedly said that authorities were trying to find links between Farook and a terrorism ring prosecuted in 2012. FBI believes that Farook had ties to a group of jihadis in California arrested for attempting to travel to Afghanistan to join al Qaeda.

“Those individuals were planning to go overseas and fight, go through Taliban training camp, and ultimately enter an al Qaeda training camp and fight against U.S. and coalition forces in Afghanistan,” Bowdich said, according to the New York Times.

Earlier on Thursday, FBI Director James Comey and other senior U.S. officials briefed members of Congress -- behind closed doors -- on the investigation. Federal officials have reportedly raised concerns over the alleged failure of authorities as they did not pick up on extremist messages exchanged during the online courtship two years ago between American-born Farook, and Malik, his then-fiancée in Pakistan.

"Everyone's asking the same questions about how it is that law enforcement didn't know, or intelligence officials didn't know -- that they could have flown under the radar and nothing gave an indication that they were a threat," said Rep. Jim Langevin, a Rhode island Democrat and member of the House Homeland Security Committee, according to the Associated Press.

According to federal government sources, investigators believe that Farook and Malik were planning an even bigger attack, the Los Angeles Times reported, adding that they were in the final planning stages of the assault on a location or building that housed more people. Investigators have based that conclusion on evidence left behind on the couple’s computers and digital devices, one of the sources reportedly said.