Both killers in last week's San Bernardino, California, shooting "were radicalized and have been for quite some time," said David Bowdich, assistant director of the FBI's Los Angeles field office. "The question for us is how, and by whom, and where were they radicalized? Maybe there's not a by whom," Bowdich told reporters Monday. "Oftentimes it's on the Internet, and we don't know."
So far, he 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."
The killers participated in target practice at ranges in the Los Angeles area before the shooting, Bowdich said. In at least one occasion, he said, the target practice occurred within days of the shooting.
[Previous story, posted at 2:30 p.m. ET Monday]
Last week's mass killing in San Bernardino, California, was an act of terrorism, law enforcement officials and President Barack Obama agree.
The 14 deaths at a holiday party left the community shaken, but it's become apparent that for shooters Syed Rizwan Farook and Tashfeen Malik, the mass shooting was motivated by political ideologies.
On Sunday night, Obama called the attack "an act of terrorism, designed to kill innocent people."
Over the weekend, new details came to light indicating that Farook and Malik, who were husband and wife, supported the ideology of ISIS and were radicalized.
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 the couple became radicalized and planned and executed the attack on their own.
Since the shootings, the FBI has found evidence of jihadist sympathies. The question is, could anything have been done to foil the plot?
As Obama said, there is no evidence that Farook and Malik were part of a larger national or international conspiracy. And their targets were not likely ones: They were Farook's co-workers.
Farook targeted his colleagues Wednesday at a holiday party for the environmental health department, killing 14 people and leaving 21 wounded.
Authorities gunned down the couple hours later in a shootout in San Bernardino.
'I cannot forgive myself'
Farook was "fixated" on Israel and supported ISIS' ideology of establishing an Islamic caliphate, his father told an Italian newspaper.
Before the shooting, the younger Farook had expressed some troubling beliefs, his father told La Stampa newspaper.
"He said he shared the ideology of (Abu Bakr) al-Baghdadi to create an Islamic state, and he was fixated on Israel," the elder Farook said, referring to the ISIS leader.
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.
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.
As more reports emerged on the shooters' fascination with ISIS, Obama spoke to the nation Sunday night in a bid to temper growing anxiety.
"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.
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 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.