An examination of digital equipment recovered from the home of the couple who killed 14 people in San Bernardino last week has led FBI investigators to believe the shooters were planning an even larger assault, according to federal government sources.

Investigators on Thursday continued to search for digital footprints left by Syed Rizwan Farook and Tashfeen Malik, scouring a downtown San Bernardino lake for electronic items, including a hard drive that the couple was hoping to destroy, sources told the Los Angeles Times.

FBI agents will probably spend days searching Seccombe Lake and canvassing the neighborhood for clues after receiving a tip that the couple may have visited the area on the day of the attack, according to David Bowdich, assistant special agent in charge of the FBI's Los Angeles field office.

Farook and Malik were in the final planning stages of an assault on a location or building that housed a lot more people than the Inland Regional Center, possibly a nearby school or college, federal sources familiar with the widening investigation said.