Shooters Syed Rizwan Farook and Tashfeen Malik practiced at gun ranges in the Los Angeles area, said David Bowdich, assistant director of the FBI's Los Angeles field office. And on at least one occasion, he said, their target practice occurred within days of the shooting.
They "were radicalized and have been for quite some time," Bowdich said.
"The question for us," he told reporters Monday, "is how, and by whom, and where."
While the couple supported ISIS, investigators are still trying to find out if either of them ever actually met any ISIS
leaders or took orders from anyone.
It is possible they became radicalized and planned and executed the attack on their own.
"Remember, oftentimes, it's on the Internet. We just don't know," Bowdich said. "I don't want to speculate."
So far, Bowdich said, investigators haven't found any evidence of a plot for the attack extending outside the continental United States.
"Right now, we're looking at these two individuals," Bowdich said, "and we are beginning to focus, to build it out from there."
Raid at family home in Pakistan
Sources told CNN that investigators believe Malik was radicalized at least two years ago, well before she came to the United States with Farook on a fiancee visa and before ISIS proclaimed its caliphate.
Authorities are looking into whether she pushed her husband to adopt more extremist views.
"We are working with our foreign counterparts to determine as much as we can," Bowdich said.
Thousands of miles away from the San Bernardino shooting scene, in a city in central Pakistan, authorities have raided a home owned by Malik's father, a security source told CNN Monday.
The forces broke a padlock off the building in the city of Multan, removing items from the unoccupied home, the source said.
'I cannot forgive myself'
Relatives didn't see any red flags, according to David Chesley, an attorney representing Farook's family.
"The family was completely surprised and devastated. ... No one had any knowledge. If anybody would have, they definitely would have done something to stop it," Chesley told CNN's "Erin Burnett: Outfront" on Monday.
Farook's mother, who shared a home with the couple and their 6-month-old baby, lived in an isolated part of the house, Chesley said.
Over the weekend, Farook's father told an Italian newspaper that his son supported ISIS' ideology of establishing an Islamic caliphate.
"He said he shared the ideology of (ISIS leader Abu Bakr) al-Baghdadi to create an Islamic state, and he was fixated on Israel," the elder Farook told La Stampa newspaper.
Chesley told CNN Monday that the father was on medications and didn't recall making those comments to the Italian newspaper.
In the newspaper interview, the father, also named Syed Farook, recalled the first time he saw his son with a gun.
"I became angry. In 45 years in the United States, I yelled, 'I have never had a weapon.' He shrugged his shoulders and replied, 'Your loss,' " the father said.
"I cannot forgive myself. Maybe if I had been at home, I would have found out and stopped him," he told the newspaper.
The attorney described the newspaper report as "doubtful at best." Asked whether the comments quoted were true, he reiterated that his client didn't recall saying them.
As more reports emerged on the shooters' fascination with ISIS, Obama spoke to the nation
Sunday night in a bid to temper growing anxiety.
He called the San Bernardino attack "an act of terrorism, designed to kill innocent people."
"The FBI is still gathering the facts about what happened in San Bernardino, but here's what we know: The victims were brutally murdered and injured by one of their co-workers and his wife," he said. "So far we have no evidence that the killers were directed by a terrorist organization overseas, or that they were part of a broader conspiracy here at home."
The gravity of the occasion was underscored by Obama's decision to address the nation from the Oval Office for only the third time in his presidency, following addresses on the Gulf of Mexico oil spill and the end of the Iraq War in 2010.
ISIS hails couple as 'supporters'
A day before Farook's father made news,
ISIS hailed the couple as "supporters" of the terror group. The FBI has said it is treating the attack as an act of terrorism.
The couple's motivation for the attack is a key focus for investigators.
Malik had posted to Facebook a pledge of allegiance to ISIS leader Baghdadi, three U.S. officials familiar with the investigation said.
But ISIS' acknowledgment of the couple as supporters doesn't mean they were members or that someone from the group ordered the massacre, said Rick Francona, a CNN military analyst and a former intelligence officer.
ISIS, when claiming responsibility for other terrorist attacks, would call attackers "knights" or "soldiers" rather than supporters. It has, however, urged sympathizers to carry out attacks on their own.
"What they're calling these two are supporters, which is kind of a lesser level," indicating ISIS might not have had direct contact with the couple, Francona said.
Link to other terror groups?
Farook looked into contacting terrorist groups overseas
, such as al Qaeda affiliate al Nusra Front and Al-Shabaab, a senior law enforcement official said.
The source did not specify when or how those attempts were made. A working theory among investigators is that Malik was radicalized before meeting her husband.
The source said at the very least, it appears ISIS and possibly other terrorist groups inspired the couple.
Officials caution there is still a lot to learn and a plethora of electronic media to review. Part of what is slowing the process down is that the couple's attempts to destroy their electronics made it challenging for investigators to use the material.
"They covered their tracks pretty well," the official said.
San Bernardino open for business
"Our hearts are heavy during this time, yet we must move forward," James Ramos, chairman of the San Bernardino County Board of Supervisors, said at a news conference Monday.
The sadness over the attack remains, Ramos said, but San Bernardino will resume its duties, with all government offices open, he said.
Employees for the environmental health services department, where the victims of the attack worked, will resume work December 14, county officials said.
Additional security guards have been posted at county offices and other security measures are in place, Ramos said.
As county leaders spoke about not letting fear interfere with daily life, some of the doctors who tended to the wounded recounted how nightmarish that day was.
"To see something of this magnitude is unexpected, to have it occur in our county is unexpected," Dr. Sakona Seng said. "It sort of tests your faith in humanity in some respects to hear that this happened."
All of the victims who made it to a hospital are in stable condition, Dr. Dev Gnanadev said. For him, the deepest pain is over those victims whom doctors didn't even get the chance to try to save.
"What really bothers me most is that none of the 14 who perished had a chance," he said.