SAN BERNARDINO, Calif. -- 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 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 Wednesday by trying to do as they had been trained - dropping under the tables and staying quiet so as not to attract attention.

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"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 U.S. 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 the Islamic State group on Facebook, authorities said. The Muslim couple were killed hours later in a gun battle with police.

"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.

John Galletta, an instructor at Riverside Magnum Range, said in a statement that Farook had been there Nov. 29 and 30, two days before the attack, and "nothing was out of the ordinary regarding his behavior."

Galletta told reporters he never spoke to Farook and 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, Galletta replied, "How are you able to determine what somebody's intents are?"

CBS News correspondent Carter Evans at Riverside Magnum Gun Range with employees Mike McGee, left, and John Galletta, right CBS News

"It is devastating to people to know that this is where he might have prepared for those last days," Galletta told CBS News correspondent Carter Evans.

Range employee Mike McGee said Farook had an AR-15.

He told Evans nothing stood out about Farook.

McGee said Farook asked him a question when his gun began smoking.

McGee said Farook "tells me that it was a new rifle. He was not familiar with it."

A federal law enforcement source told CBS News investigative correspondent Pat Milton Enrique Marquez, who purchased the two assault rifles used in the attacks, was questioned by the FBI Monday. The source said Marquez was apparently providing information. He had not been arrested or charged.

The FBI couldn't interviewed Marquez until Monday because he was being evaluated at a mental health facility where he had checked himself in after the shooting.

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.

A custody hearing for Farook and Malik's 6-month-old daughter was held 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.

President 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.

Meanwhile, most of the county's 20,000 employees went back to work Monday 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 tightened security at county buildings and offered counseling and a hotline for employees in distress.

Employees in the environmental health division, where Farook and many of his victims worked, will be off until next week. It was the environmental health division that held the active-shooter training a year ago.

"We held each other and we protected each other through this horrific event," said county Health Director Trudy Raymundo, who was in the room during the attack, "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 where the return to work was announced 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."