More than a week later, authorities are still digging to make sense of the San Bernardino massacre, piecing together the clues they've compiled so far and trying to discover more, including by investigating the shooters' electronic trail and searching a lake near where the carnage occurred.
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.
These are the latest and most significant investigative leads so far:
Friend of killer says he helped build prior pipe bombs
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.
He previously said that he and Farook conceived a terror plot in 2012, but abandoned their plans after the FBI arrested an unrelated group in Riverside.
Marquez has emerged as a person of interest in the San Bernardino shootings because he bought rifles used in the attack.
He checked himself into a mental health facility after the San Bernardino 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. Investigators believe the hard drive was removed as a way to cover up the crime.
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.
Previously this week
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.
Online communications recovered so far indicate that the two killers had become radicalized long before carrying out last week's attack.
The FBI is still trying to recover information from two smashed cell phones found in a garbage can near the home.
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."