SAN BERNARDINO, Calif. (CNN) — Syed Rizwan Farook and Tashfeen Malik were radicalized, with the latter pledging allegiance to ISIS while the pair killed 14 people in San Bernardino, California.

But when, and how, did they take this dark turn?

Officials are trying to figure that out, as well as who might have guided or helped the married couple turn their extreme views into a horrific act of terrorism, as characterized by President Barack Obama and law enforcement officials.

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.

Still, it's hard to say what precipitated her or Farook's shift, with one U.S. official noting, "It's not like a switch goes on and you're radicalized."

"It's complicated," said another U.S. official. "They were looking to be radical and attach themselves to (a group)."

Investigators are working to determine if the pair ever met or took orders from ISIS leaders, or if anyone outside the United States had a hand or even knew of their plans. It's possible they did everything -- from becoming radicalized to planning and executing the attack -- on their own.

"Remember, oftentimes, it's on the Internet," David Bowdich, assistant director in charge of the FBI's Los Angeles field office, said of how terrorists get their inspiration and ideas. "We just don't know."

Sources: Authorities raid Malik's former home

Born in Chicago, Farook moved and spent most of his life in California. Police shot and killed him and his wife in a shootout shortly after the San Bernardino massacre Wednesday.

Malik, though, hadn't been in the United States for long.

She grew up in central Pakistan. On Monday, Pakistani intelligence authorities raided the home owned by her father in Multan, a city about 220 miles (350 kilometers) southwest of Lahore. Malik had lived there until spring 2014, right around the time she got married and moved to the United States on a fiancee visa.

After finding the residence padlocked and chained shut, authorities got inside and seized religious instruction books, audio CDs with Quran readings and various documents, according to Pakistani intelligence sources.

Malik took but never completed a Quranic course through the Al-Huda International Welfare Foundation. In a statement, that religious study center said that she told her instructor in May 2014 that she wouldn't be able to finish because she was about to get married.

And Abdul Aziz, the main cleric of the Red Mosque in the Pakistani capital of Islamabad, "denied any link or connection" to Malik and denied that he'd had pictures taken with her.

Father says Farook backed ISIS; lawyer 'doubtful'

"(Farook's) family was completely surprised and devastated," David Chesley, a family lawyer, said Monday about the attack. "... No one had any knowledge. If anybody would have, they definitely would have done something to stop it."

Farook's mother, who shared a townhouse in Redlands, California, 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 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.' The son 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.

Chesley, the Farook family attorney, described the newspaper report as "doubtful at best."

Official: 'They covered their tracks'

No one is denying that Farook and his wife opened fire at a holiday luncheon for his co-workers at the Inland Regional Center on Wednesday. One question is who influenced who, including whether Malik may have pushed her husband to think and act the way he did.

The couple did practice shooting together at gun ranges in the Los Angeles area, according to Bowdich. In fact, they had target practice at least once within days of the massacre that left 14 dead, the FBI official said.

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. At the very least, it appears that ISIS and possibly other groups inspired the couple, according to the official.

Days after Malik's Facebook pledge to its leader Abu Bakr al-Baghdadi, ISIS hailed her and her husband as "supporters" of the terror group.

But this acknowledgment doesn't mean that the pair were members or that someone from the group ordered the massacre, said Rick Francona, a CNN military analyst and former intelligence officer. ISIS has urged sympathizers to carry out attacks on their own, though it typically refers to those who take orders directly from its leaders as "knights" or "soldiers" rather than supporters.

Officials caution there is still a lot to learn and much electronic media to review. Part of what is slowing the process 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.

— CNN's Brian Todd, Pam Brown, Kyung Lah, Sophia Saifi and Joe Sutton contributed to this report.

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