The couple suspected of killing 14 people in California this week had stockpiled thousands of bullets and bomb-making equipment at home - fuelling talk the rampage was a terrorist act and raising the possibility the pair had been plotting a broader attack.
The motive for the mass shooting is not yet clear and no evidence has directly linked the two suspects - Syed Rizwan Farook and his wife Tashfeen Malik - with Islamic State in Iraq and Syria or any other organisation, but the FBI has not dismissed terrorism.
There was no hard evidence the duo had been radicalised but officials quoted by CNN said Farook had been in contact with known terror suspects overseas and had become radicalised after marrying Malik in Saudi Arabia last year. An imam at a local mosque he attended said Farook showed no signs of that, CNN said.
The FBI, which has assigned counter-terrorism agents and is scouring the couple's electronic devices, had evidence Farook had communicated with extremists, domestically and abroad, a few years ago, but not recently, The New York Times reported, citing congressional officials who were briefed on the investigation.
"We do not yet know the motive, we cannot rule anything out at this point," Mr David Bowdich, assistant director of the FBI's Los Angeles office, told a news conference on Thursday. "There was obviously a mission here. We know that. We do not know why."
United States President Barack Obama, who has repeatedly called for tougher gun control measures, said that while the rampage that also injured 21 people at a social services centre on Wednesday might have terrorism links, it could also be workplace-related.
"We do know that the two individuals who were killed were equipped with weapons and appeared to have access to additional weaponry at their homes. But we don't know why they did it. We don't know at this point the extent of their plans. We do not know their motivations," he said on Thursday.
But Chief Jarrod Burguan of the San Bernardino police said the killing spree seemed more than a spontaneous workplace dispute.
"There appears to be a degree of planning that goes into this," the chief said at a news conference. "Nobody goes to a party and puts together such an elaborate scheme."
Chief Burguan gave some grim numbers from the country's worst mass shooting in almost three years.
At the Inland Regional Centre, Farook and Malik fired between 65 and 75 bullets and left behind a bundle of three pipe bombs connected to a remote control car.The bombs failed to go off.
The authorities estimate between 75 and 80 people were at the Christmas party when the couple attacked.
During the subsequent shoot-out with police, the duo fired another 76 rounds at officers. The police in turn, fired some 380 at the assailants, killing them both.
They subsequently found nearly 1,600 bullets either on the attackers or in their getaway car.
At their townhouse, police uncovered another 4,500 bullets, 12 pipe bombs and tools and materials to make more. "Clearly they were equipped and they could have done another attack," Mr Burguan said.
Neither was on any terror watchlist nor did they have criminal records. Family, friends and co-workers also said they had not noticed anything different about the couple.
Dr Danny Davis, a homeland security expert at Texas A&M University's Bush School of Government and Public Service, told The Straits Times that some of the puzzling circumstances surrounding the attack might be because the attackers had mixed motives and never intended to attack the Inland Regional Centre.
"Maybe he (Farook) was provoked at the Christmas party. And so even though they had plans, they decided it was time to do it now. The investigation will prove whether that is true or not but that is what it seems like to me," he said.
He added that the authorities would refrain from declaring a terrorist attack until they have absolute proof so as not to raise alarm unnecessarily.
"The FBI will probably be more comfortable calling it a terrorist attack if they have something firm. Calling it a terrorist attack connects it to a bigger cause."