Farook's father talked to an Italian newspaper about his son, one of two shooters in last week's deadly attack in San Bernardino.
Along with his wife, Tashfeen Malik, Farook targeted his co-workers 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.
'I cannot forgive myself'
Before the shooting, 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.
As more reports emerge on the shooters' fascination with ISIS, President Barack Obama spoke to the nation Sunday night in a bid to temper growing anxiety.
He described the attack as "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 use the symbolic power of 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 the father made news, ISIS hailed the couple as "supporters" of the terror group. The FBI has said it was 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 al-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 it, 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 it 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 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 it's too early to put a label on the attack, but at the very least, it appears the couple was inspired by ISIS and possibly other terrorist groups.
Officials caution there is still a lot to learn and there is 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 exploit the material.
"They covered their tracks pretty well," the official said.
Long road to normalcy
San Bernardino worked to return to some sort of normalcy Monday with the reopening of most county offices shut down after the massacre. Employees for the environmental health services, where the victims of the attack worked, will resume work December 14, county officials said.
"Security will be increased at public health buildings throughout San Bernardino to provide added peace of mind," San Bernardino County said in a statement.