THE man suspected of opening fire and killing 14 people in San Bernardino had reportedly been in touch with more than one international terrorism subject who the FBI were already investigating.
CNN reported that law enforcement sources said Syed Rizwan Farook had been in touch with them on social media and by telephone, suggesting that he may have become radicalised in the months before the attack.
Police said Mr Farook and his wife, Tashfeen Malik, fired between 70-75 rounds as they entered the Inland Regional Centre in San Bernardino on Thursday morning. The couple - armed with semi-automatic rifle and hand-guns - also left a pipe bomb at the building.
When they were later shot and killed by officers, they were found to be in possession of 1,400 .223 ammunition and 200 9mm rounds.
At a house in the nearby town of Redlands associated with the couple, police found more than 2,500 round of .223 ammunition, 2,000 bullets for the hand guns and 12 pipe bombs.
They also found tools that could have been used for the production of more bombs.
"At this point we do not know a motive," said San Bernardino Police Chief Jarrod Burguan.
Asked about Mr Farook's possible contact with international terror suspects, FBI official Mark Bowdich said the international angle was still being looked at that angle.
He said the couple had entered the US in 2014 and that inquiries were underway into overseas visits that had made - reportedly to Pakistan and Saudi Arabia.
"It would be premature to call it terrorism at this point," he said.
On Thursday, President Barack Obama stressed that the FBI was still investigating the incident and had not yet come to a conclusion as to the motive that lay behind the shooting.
Mr Obama, who ordered that US flags to be flown at half-staff to honour the victims of the shooting, said it was possible the motive was mixed.
"It is possible that this was terrorist-related, but we don't know. It's also possible this was workplace-related," Mr Obama said after a meeting with his national security team.
"We don't know why they did it. We don't know at this point the extent of their plans."
Mr Obama repeated his case for new gun control measures - a call he has promised to issue after every mass shooting - although he did not lay out specific proposals or criticise the Republican-led Congress.
"We all have a part to play. As the investigation moves forward, it's going to be important for all of us, including our legislatures, to see what we can do to make sure that when an individual decides they want to do somebody harm, we make it a little harder for them to do it. Right now, it's just too easy.
"We're going to have to, I think, search ourselves as a society," he said.