The identity of the killers, Syed Rizwan Farook and Tashfeen Malik, is not in doubt. But investigators want to find out more about who this married couple interacted with, how they hatched and carried out the plot, and why.
Enrique Marquez, a former neighbor of Farook, told investigators he and his friend previously built pipe bombs, law enforcement officials said.
Marquez said he had nothing to do with devices found at the home of Farook and Malik, or the ones that apparently failed to go off at the site of the shooting, the officials said.
Marquez, who has spent several days being voluntarily interviewed by the FBI, portrayed the two men as hobbyists experimenting with building the devices, the officials told CNN.
He also boasted, one official said, that if he had made the bombs they would have gone off.
Marquez has emerged as a person of interest in the San Bernardino shootings because he bought rifles used in the attack. He has not been charged with a crime.
Divers search a small lake near where shooters had been, the FBI says
Divers ended Friday evening a second search of a small lake in a park a few miles from San Bernardino's Inland Regional Center, where 14 people were killed last week.
David Bowdich, the assistant director in charge of the FBI's Los Angeles office, wouldn't comment on what they were looking for. But he said Farook and Malik were in the area "at some point," without specifying if this was before or after the massacre.
One of the items divers likely are looking for is a missing hard drive from the couple's computer.
Bowdich has said the search will take several days.
Farook was in the same 'social circle' as convicted terrorist, FBI believes
The FBI believes Farook had ties to a group of jihadists in California who were arrested in 2012 for attempting to travel to Afghanistan to join al Qaeda.
Specifically, investigators are taking a new look at one of the four men who was arrested in the 2012 case, Sohiel Kabir. Kabir, who was convicted and sentenced to 25 years in prison, was the recruiter who helped radicalize others.
The 2012 probe happened in Riverside, California, not far from San Bernardino. Farook was in the social circle of Kabir, officials told CNN.
The FBI initially investigated five men as part of the Riverside group. The fifth wasn't arrested and hasn't been identified. But Farook was not among those investigated at the time, the officials said.
Marquez says Farook considered terror plot in 2012
Marquez told investigators that he and Farook conceived a terror plot in 2012, but abandoned their plans after the FBI arrested an unrelated group in Riverside that had been planning to join al Qaeda in Afghanistan.
Marquez checked himself in to a mental health facility after the San Bernardino attack.
He is cooperating with the FBI, but investigators are still trying to corroborate the information he has provided, including details of the alleged 2012 plot.
Marquez claims he knew nothing about the San Bernardino shootings. He has not been charged with a crime.
That same friend's marriage -- to a woman of Russian descent -- is under scrutiny
Now that Farook's friend and former neighbor Enrique Marquez has the attention of investigators, his own relationships are being scrutinized.
Federal authorities are "examining the circumstances of the marriage" between Marquez and a woman of Russian descent, according to a source familiar with the matter. Investigators are looking into whether this was a sham marriage for immigration purposes.
The woman came to the United States from Russia on a J-1 visa in 2009, according to another law enforcement official. That official said the woman did not appear for an interview earlier this week for her permanent resident application.
The FBI has recovered some electronic communication from killers, officials say
Farook and Malik tried to destroy their electronic tracks, but the FBI has been able to recover some of those communications, two law enforcement officials told CNN.
FBI Director James Comey told a Senate hearing Wednesday the agency has found online discussions about jihad between Farook and Malik from late 2013, before they began dating. Investigators have found data on a tablet computer and other cell phones at the couple's home, he said.
The FBI is still trying to recover information from two smashed cell phones found in a garbage can near the home. They also have not found the hard drive from a computer at the home; investigators believe the hard drive was removed as a way to cover up the crime.
The shooters talked about jihad and martyrdom before marrying, FBI leader says
The online communications recovered so far indicate that the two killers had become radicalized long before carrying out last week's attack.
Comey elaborated a bit on the conversations between Farook and Malik, where they talked to each other about jihad and martyrdom before they were married and before Malik immigrated to the United States.
Asked if Malik was radicalized before she entered the United States, Comey replied: "So far, it looks like she was... she was before she connected with the other killer."
Authorities say Farook's friend, Marquez, bought weapons used in the rampage
The two AR-15s used in the San Bernardino attack were purchased by Enrique Marquez for Farook back in 2011 and 2012, two law enforcement officials said. Marquez acknowledged that he bought the weapons and soon after gave them to Farook, though he never reported the ownership transfer. Such transactions could be a violation of California law, the officials said.
Farook borrowed $28,500 via online lender Prosper last month
Farook took out a loan for $28,500 in November, multiple law enforcement officials told CNN. Investigators have accounted for all the money and do not believe any of the money 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, from the online lender Prosper, is not considered of significant investigative value at this stage.
About half was given to Farook's mother in the last couple of weeks, one official said and some was spent on household items.
It is believed the couple had bought the arsenal they used before the loan was taken out.
A representative for Prosper would not confirm the existence of the loan, but provided CNN the following statement: "Prosper is prohibited by law from disclosing any non-public, personally identifiable information regarding any loan originated through our platform. All loans originated through the Prosper platform are subject to all identity verification and screening procedures required by law, including US anti-terrorism and anti-money laundering laws. As part of our standard procedures, we also confirm that all loan funds are disbursed into a verified US bank account in the borrower's name. Like all Americans, Prosper is shocked and saddened by recent events in San Bernardino."
A man thought to be Farook went to a gun range -- alone -- days before the attack, a source says
A man believed to be Farook visited the Magnum Shooting Range in Riverside, California, alone on the Sunday and Monday prior to the December 2 attacks in San Bernardino, according to a source familiar with the matter.
He practiced shooting an AR-15. The source said sign-in logs at the range were paired with surveillance video that have been turned over to the FBI. The FBI was at the range Monday night asking questions and showing pictures of another male (not Farook), asking if that man had ever been to the range.
Officials: While the shooting was happening, Malik pledged allegiance to ISIS in a Facebook post
As the San Bernardino attack was happening, investigators believe Malik posted on Facebook pledging allegiance to ISIS leader Abu Bakr al-Baghdadi, three U.S. officials familiar with the investigation told CNN.
Malik posted the comment, but it was made on an account with a different name, one U.S. official said. The officials did not explain how they knew Malik made the post.
"There was a post that went up around 11 a.m. PT on the account Wednesday that violated the company's community standards (those standards prohibit people using Facebook to promote terrorism or celebrate/glorify violence)," a Facebook official told CNN. "It was taken down Thursday. He said he couldn't go into details about the nature of the post other than it violated standards (pointed to their standards prohibiting terrorism). The official said Facebook is cooperating with law enforcement."
Authorities think the couple met in 2013 in Saudi Arabia during the Hajj
Farook traveled to Saudi Arabia in 2013 for the Hajj for a couple of weeks, two government officials said.
One of the officials explained that this is a common occurrence and the trip would not have raised any red flags as many Americans travel to Saudi Arabia during that time.
The feds believe Farook met Malik during that trip. Malik eventually came to the United States on a fiancée visa, the official said. She then obtained a green card.