As the FBI officially labeled the investigation of Wednesday's mass shootings in San Bernardino, California as a terrorism probe, the FBI chief said there was no evidence as yet that the husband and wife team was part of a larger operation.

"There is no indication that they are part of a network," said FBI Director James Comey in a meeting with reporters.

"So far, we have no indication that these killers are part of an organized larger group, or form part of a cell," Comey added.

Comey said the two seem to have been inspired from afar, as he acknowledged that Syed Farooq and Tafsheen Malik were not on the radar of the FBI.

"We are spending a tremendous amount of time, as you might imagine, over the last 48 hours trying to understand the motives of these killers and trying to understand every detail of their lives, Comey told reporters.

"There is much about this that doesn't make sense, even to those of us who do this for a living," the FBI chief acknowledged.