Officials say it could have been years before last week's deadly attack in San Bernardino, California.
Investigators believe shooter Syed Rizwan Farook
may have been plotting an earlier attack in California with someone else, two U.S. officials told CNN on Tuesday
. One of the officials said the two conspired in 2012 and a specific target was considered. The pair "got spooked" and decided not to go through with the earlier attack after a round of terror-related arrests in the area, an official said.
And sources also told CNN that investigators believe Tashfeen Malik
, Farook's wife and the other shooter who opened fire Wednesday, was radicalized at least two years ago.
Investigators are still trying to piece together profiles of the couple who killed 14 people and died in a gunbattle with police last week. They're also working to pinpoint whether anyone in the United States or abroad helped finance and shape the plot.
Farook took out a bank loan for $28,500 in November, multiple law enforcement officials told CNN on Tuesday.
About half the money was given to Farook's mother in the last couple of weeks, one official said, and some of it was spent on household items.
Investigators have accounted for all the money and do not believe any of it was provided to the killers by any outside entity backing the plot, according to one of the sources. Because of that, the officials said the loan is not considered of significant investigative value at this stage.
But one expert told CNN the loan could be another sign that Farook had been preparing for the attack.
"What it indicates is he was financing this operation or his life or his afterlife for his child and mother, using what is now wire fraud and bank fraud, so it's just two more additional charges that the FBI will be looking at," said Tim Clemente, a former FBI counterterrorism agent.
Sources: Authorities raid Malik's former home
The final post on a Facebook page believed to be associated with Malik used the word "we" and pledged allegiance to ISIS, an indication, a U.S. official said, that it was a statement on behalf of both killers.
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 in or knew of their plans. Officials say it's possible the husband and wife did everything, from becoming radicalized to planning and executing the attack, on their own.
Sources told CNN that investigators believe Malik was radicalized well before she came to the United States with Farook on a fiancee visa and before ISIS
proclaimed its caliphate.
It's hard to say exactly what caused the couple's views to shift.
"It's complicated," said another U.S. official. "They were looking to be radical and attach themselves to (a group)."
Malik grew up in central Pakistan and studied pharmacy at a university there. 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, 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 she told her instructor in May 2014 that she wouldn't be able to finish because she was about to get married.
Officials there condemned the attack and said there were no signs Malik had developed an extremist mindset.
"No one could ever think she could do such a heinous thing," Al-Huda spokeswoman Farrukh Choudhry said.
Over the weekend, Farook's father told an Italian newspaper that his son supported ISIS' ideology of establishing an Islamic caliphate. An attorney for the family told CNN the father was on medication and didn't recall making those comments.
Target practice before attack
No one is denying that Farook and his wife opened fire on his co-workers at the Inland Regional Center on Wednesday. One question is who influenced whom, including whether Malik may have pushed her husband to think and act the way he did.
The couple practiced shooting at gun ranges in the Los Angeles area, according to David Bowdich, assistant director of the FBI's Los Angeles field office.
A man believed to be Farook visited the Magnum Shooting Range in Riverside, California, on the Sunday and Monday prior to the attack, according to a source familiar with the matter. On those days, he shot with an AR-15.
"He presented what appeared to be a valid ID, came in and acted ... the way normal people act," said John Galletta, an instructor at the gun range.
Farook was alone on his recent visits to that range, the source said. But the FBI was at the range Monday night asking questions and showing pictures of another man, asking if he'd ever been there.
Surveillance video and logs from the range have been turned over to the agency.
Official: Former neighbor gave shooter weapons
Enrique Marquez purchased two AR-15s used in last week's attack for Farook in 2011 and 2012, two law enforcement officials said Tuesday.
Marquez, a former neighbor of Farook, gave him the weapons but never reported the ownership transfer. Such transactions could be a violation of California law, the officials said.
Marquez has been cooperating with law enforcement and so far, authorities do not believe he had any involvement in the shooting plot, law enforcement officials said.