Investigations into the San Bernardino shooting carried out by a radicalised couple has expanded overseas including Pakistan, a top US official has said.

"Pakistan is one of those countries, there are other countries also involved...," Loretta Lynch, US Attorney General told NBC's 'Meet the Press' in an interview yesterday.

The Federal Bureau of Investigation is leading the probe into the killing of 14 people by Pakistani national Tashfeen Malik, and her husband, Syed Rizwan Farook, of Pakistan origin on Wednesday. The US President has described this as an act of terror.

"This investigation is ongoing. It's wide ranging, It's very complex. ... This is an FBI investigation now because of the indications that we do have of radicalisation. This is a terrorism investigation at this point in time but we continue to work closely with our state and local counterparts, also ATF Marshals are on the ground," Lynch said in response to a question.

Tashfeen and Farook married in Saudi Arabia some two years ago after they initially came in contact over a matrimonial site. Tashfeen, who spent most of her life in Saudi Arabia, was in Pakistan to complete her studies, before coming to the US as a fiance of Farook, a US national of Pakistani origin.

"We're all sort of really focusing on the victims of this horrific attack," the US attorney said adding that in the four days since the attack took place, investigating authorities have done more than 300 interviews and have searched several locations.

"A lot of information being processed, being analysed and being gathered and more to come. So, what I would say to people is that this investigation as it has already been stated is a marathon, not a sprint, but it is one of great concerns to the American people and so we're committed to keeping people informed," she said.

The investigations so far, she said, has not thrown indication of why the couple picked up guns and become murderers. "I can say that is the focus of our investigation. We're looking at everything we can find out about these two killers' lives, how they grew up, where they grew up, how they met. All of those things will provide us guidance," she said.

Lynch said the US is also talking with Saudi authorities. "I'm not able to go into a lot of specifics there is because we are working closely with our foreign counterparts to gather that information. It will be a long process, it will be an exhaustive process and we are trying to learn as much as we can about her life before they met, after they met, and, frankly, after she came here as well," she said.

"What we are trying to focus on again is what motivated these two individuals. What led them not just to commit the act but to pick this particular place?" the US attorney general said.