The FBI said Thursday it was too soon to say what motivated a young couple to open fire on a holiday party in California, killing 14 people, adding it was premature to label it terrorism.

"We cannot rule anything out at this point," David Bowdich, FBI assistant director in Los Angeles, told reporters. "There was obviously a mission here. We know that. We do not know why."

"It would be irresponsible of me to call this terrorism. The FBI defines terrorism very specifically. That is the big question for us, is what is the motivation for this."