Farook and his wife, Tashfeen Malik, opened fire at Farook's co-workers at a holiday party for the environmental health department before being gunned down hours later in a shootout with authorities.
When asked by reporter for the La Stampa newspaper whether his son had ever talked about ISIS, Farook's father said that he had.
"Who does not talk about it these days? He said he shared the ideology of al-Baghdadi to create an Islamic State, and he was fixated on Israel," the elder Farook said, referring to ISIS leader Abu Bakr al-Baghdadi.
He recalled a time he saw his son -- who he described as studious, hardworking and obedient -- 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 Italian newspaper.
The news was reported Sunday, one day after ISIS hailed Farook and Malik as "supporters" of the terror group and two days after the FBI said it was treating the attack as an act of terrorism.
On Friday, three U.S. officials familiar with the investigation told CNN that Malik posted to Facebook a pledge of allegiance to ISIS leader al-Baghdadi.
The couple's motivation for the attack is a key focus for investigators.
But ISIS' acknowledgment of Malik and Farook as supporters doesn't mean they were members or that someone from the group ordered it, said retired Air Force Lt. Col. 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, said Francona.
A senior law enforcement official told CNN that Farook looked into contacting terrorist groups overseas, such as al Qaeda affiliate al Nusra and Al-Shabaab. 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 such as al Qaeda.
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 have made it challenging for investigators to exploit the material.
"They covered their tracks pretty well," the official said.
A church grieves
Among the 14 victims of last week's shooting was 27-year-old Yvette Velasco, a regular at St. Catherine of Siena Church in Rialto, California.
The Rev. Rogelio Gonzalez spoke Sunday about the church's loss.
"This week, we have been witness on how evil can touch anyone, anywhere, any given moment," he said. "There's questions. How can something like this take place? How can I reconcile my faith with what's happening?"
Velasco leaves behind her parents and three sisters.
"Yvette was an intelligent, motivated, and beautiful young woman, who was full of life and loved by all who knew her," her family has said in a statement.
The President speaks
In a bid to address the growing anxiety about ISIS, President Barack Obama spoke to the nation
He characterized what happened in San Bernardino 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. 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.
"But it is clear that the two of them had gone down the dark path of radicalization, embracing a perverted interpretation of Islam that calls for war against America and the West," the President said.
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.