The San Bernardino killers had been radicalized "for quite some time" and had taken target practice at area gun ranges, in one instance just days before the attack that left 14 people dead, the FBI said Monday.
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 very conference room where he and his wife opened fire on them last Wednesday.
It was not immediately clear whether Farook attended the late-2014 training session on how to react to a workplace gunman, San Bernardino County spokeswoman Felisa Cardona said. It was held for members of the department's environmental health division, where Farook was a restaurant inspector, she said.
On Monday, two employees who had been in the room during the attack on a holiday luncheon said colleagues tried to do just as they had been trained — drop under the tables and stay 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 born in the U.S. to a Pakistani family, and Malik, a 29-year-old immigrant from Pakistan, launched the attack at about the same time Malik pledged allegiance to the Islamic State group on Facebook, authorities said. The Muslim couple were killed hours later in a gunbattle with police.
The San Bernardino Tragedy:
"We have learned and believe that both subjects were radicalized 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."
He 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.
In addition, authorities discovered 19 pipes in the couple's home in Redlands, California, that could be turned into bombs, 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 posted by The Press-Enterprise newspaper of Riverside.
In addition to the 14 killed, 21 people were hurt. At least six remained hospitalized, two in critical condition.
President Barack Obama said in a prime-time address Sunday night that the attack was an "act of terrorism designed to kill innocent people."
The killers had "gone down the dark path of radicalization," he said, but there was no evidence they were part of a larger conspiracy or were directed by an overseas terror organization.
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.
Marquez has not been charged with a crime.
The FBI would not release details on where the husband-and-wife killers practiced their shooting.
But John Galletta, an instructor at Riverside Magnum Range, said in a statement that Farook had been there on Nov. 29 and 30, two days before the attack, and "nothing was out of the ordinary regarding his behavior."
Galletta told reporters that he never spoke to Farook and that no one had seen his wife around there.
Asked whether in hindsight he or others in the shop should have been suspicious of Farook, Galletta said: "How are you able to determine what somebody's intents are?"
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.
"To honor them, to express our gratitude for their unimaginable sacrifice, we have to fight to maintain that ordinary," County Supervisor Janice Rutherford said of the victims. "We can't be afraid of our lives, of our community, of our neighbors, of our co-workers."
Authorities said that they have tightened security at county buildings and that counseling centers and a hotline have been set up for employees in distress.
Employees in the environmental health division, where many of the victims worked, will be off until next week.
In announcing the return to work, Trudy Raymundo, county health director, recalled that she was about to give a presentation when the killers opened fire.
"We held each other and we protected each other through this horrific event," she said, "and we will continue to hold each other and protect each other."
Porter, her colleague, said neither shooter spoke before firing.
"We weren't quite sure if it was an exercise the staff were throwing that they forgot to tell us about," he said, "but we all reacted instinctively and went under our tables."
At the same news conference were some of the doctors who rushed to treat the victims.
"What really bothers me most," said Dr. Dev GnanaDev, chief of surgery at Arrowhead Regional Medical Center, "is that none of the 14 who perished had a chance."
Pritchard reported from Los Angeles. Contributing to this report were John Antczak, Sue Manning and Christine Armario in Los Angeles; and Brian Skoloff in Riverside, California.