The San Bernardino killers had been radicalised "for quite some time" and had taken target practice at gun ranges, in one instance just days before the attack, the FBI said.
In a chilling twist, authorities also disclosed that a year before the rampage, Syed Farook's co-workers at the county health department underwent "active shooter" training in the same conference room where he and his wife opened fire on them last week.
It was not immediately clear whether Farook attended the late-2014 session on what to do when a gunman invades the workplace, San Bernardino County spokeswoman Felisa Cardona said.
Two employees who survived the attack said colleagues reacted by trying to do as they had been trained - dropping under the tables and staying quiet so as not to attract attention.
"Unfortunately, the room just didn't provide a whole lot of protection," said Corwin Porter, assistant county health director.
Farook, a 28-year-old restaurant inspector born in the US to a Pakistani family, and Tashfeen Malik, a 29-year-old immigrant from Pakistan, went on the rampage at a holiday luncheon at about the same time Malik pledged allegiance to Islamic State on Facebook, authorities said.
The Muslim couple were killed hours later in a gunbattle with police.
"We have learned and believe that both subjects were radicalised and have been for quite some time," said David Bowdich, chief of the FBI's Los Angeles office.
He added: "The question we're trying to get at is how did that happen and by whom and where did that happen? And I will tell you right now we don't know those answers."
Mr Bowdich also said the couple had taken target practice at ranges in the Los Angeles metropolitan area, with one session held within days of the rampage.
John Galletta, an instructor at Riverside Magnum Range, said in a statement that Farook had been there on November 29 and 30, two days before the attack, and "nothing was out of the ordinary regarding his behaviour".
Mr Galletta told reporters that he never spoke to Farook and that no-one had seen Farook's wife around there.
Asked whether in hindsight he or others at the range should have been suspicious of Farook, Mr Galletta said: "How are you able to determine what somebody's intents are?"
Authorities also discovered 19 pipes in the couple's home in Redlands, California, that could be turned into bombs, Mr Bowdich said. The FBI previously said it had found 12 pipe bombs.
Newly released emergency radio transmissions from the fast-moving tragedy show that police identified Farook as a suspect almost immediately, even though witnesses reported that the attackers wore black ski masks.
An unidentified police officer put out Farook's name because Farook had left the luncheon "out of the blue" 20 minutes before the shooting, "seemed nervous", and matched the description of one of the attackers, according to audio recordings.
In addition to the 14 killed, 21 people were hurt. At least six remained in hospital, two in critical condition.
President Barack Obama said in a prime-time address on Sunday night that the attack was an "act of terrorism designed to kill innocent people".
The killers had "gone down the dark path of radicalisation", he said, but there was no evidence they were part of a larger conspiracy or were directed by an overseas terror organisation.
The two assault rifles used in the attack had been legally purchased by an old friend of Farook's, Enrique Marquez, authorities said, but they are still trying to determine how the couple got the weapons. Mr Marquez has not been charged with a crime.
Meanwhile, most of the county's 20,000 employees went back to work for the first time since the rampage five days earlier plunged the community into shock and mourning.
A custody hearing for Farook and Malik's 6-month-old daughter was held on Monday, with Farook's sister seeking to adopt the baby, the Council on American-Islamic Relations said.
No long-term decisions were made, and the child will remain in county custody for now. Another hearing is set for next month.