In the year since Syed Rizwan Farook and Tashfeen Malik killed 14 people in a terrorist attack in San Bernardino, authorities have conducted more than 600 interviews, gathered more than 500 pieces of evidence and served dozens of search warrants.
They launched an unprecedented legal battle with Apple in an effort to unlock Farook's iPhone and deployed divers to scour a nearby lake in search of electronic equipment the couple might have dumped there.
But despite piecing together a detailed picture of the couple's actions up to and including the massacre, federal officials acknowledge they still don't have answers to some of the critical questions posed in the days after the Dec. 2, 2015, attack at the Inland Regional Center.
Most important, the FBI said it is still trying to determine whether anyone was aware of the couple's plot or helped them in any way. From the beginning, agents have tried to figure out whether others might have known something about Farook and Malik's plans, since the couple spent months gathering an arsenal of weapons and building bombs in the garage of their Redlands home.
Officials said they don't have enough evidence to charge anyone with a crime but stressed the investigation is still open.
"There are unanswered questions in this case," FBI spokeswoman Laura Eimiller said. "There is an ongoing investigation into did they get financial or material support from anyone else."
The FBI has made several public appeals to help build a timeline of the terrorist couple's movements between the attack and the beginning of a high-speed pursuit that would end with police fatally shooting them. In particular, officials said they could not account for the couple's movements during a key 18-minute period.
But after checking video surveillance and interviewing countless witnesses, the FBI said, it still can't say where Farook and Malik were during that time.
Another frustration has been the couple's electronics. Early on in the investigation, federal officials stressed that the couple's digital footprint would be key to building a complete picture of the plot.
The FBI was finally able to get a third party to unlock Farook's work-issued iPhone. But officials said it didn't yield any clues to the attack. The FBI believed the pair tried to destroy hard drives and other electronic devices, but the investigation has not yielded much on that front.
Experts said these gaps, while vexing, are far from uncommon in such sprawling investigations.
"There are always going to be untied threads," said Brian Levin, a terrorism expert and director of the Center for the Study of Hate and Extremism at Cal State San Bernardino. "And while this is most likely the work of a duo, there is always enough threads to leave open the question: What did those closest to the assailants actually know?"
Levin and others cited the 1995 Oklahoma City bombing, which killed 168 people. More than 20 years later, some questions about that terrorist attack remain unanswered.
Investigators quickly arrested Timothy McVeigh and Terry Nichols in the case. McVeigh suggested others knew about their plans, There also remains a variety of questions over the timeline of the attack and whether McVeigh really had the sophistication to mastermind such a destructive plot on his own.
Brian Jenkins, a terrorism expert and senior advisor to Rand Corp.'s president, cited the 2013 Boston Marathon bombing case as another example in which terrorism investigators pieced together many of the facts but failed to answer all the questions.
"When you are recreating the events before and after an attack, there are lot of pieces and sometimes there are gaps," Jenkins said.
Despite the lingering uncertainties in the San Bernardino terror attack, federal officials were able to lock down some key facts in the early days of the investigation.
Farook traveled to the Middle East before the attack and came back with his wife. Farook was born in Illinois, but Malik was raised in Pakistan and lived in Saudi Arabia before marrying him. From the beginning, one of the biggest questions was whether the attack was part of a larger international plot hatched by Islamic State or some other Islamic terror group. Malik posted a note on Facebook pledging allegiance to Islamic State shortly after the attack, authorities said.
The FBI concluded fairly quickly that the couple were "self-radicalized." They were inspired by terrorist groups, officials said, but did not receive financial support from any foreign or domestic organizations.
"They are essentially homegrown terrorists, self–radicalized and inspired by those overseas," Jenkins said.
A few weeks after the massacre, authorities alleged that Farook and a friend, Enrique Marquez Jr., had planned an earlier bombing and shooting plot against a community college and drivers on the 91 Freeway but aborted the idea. Marquez is accused of purchasing two military-style rifles that Farook and Malik used in the attack, which also wounded 22 people at the regional center and two police officers in the final shootout.
But federal authorities said Marquez bought the weapons years earlier and didn't know about last year's plot or participate in it.
One of the early questions for investigators was whether those closest to the couple knew what they had been planning. Family members have said Farook was outspoken about his fundamentalist views but said they had no idea he planned violence. The FBI has said it found no evidence that Farook's relatives had prior knowledge of the assault.
The question of whether others had suspicions about the couple's activities has loomed large and was an issue in the presidential race. President-elect Donald Trump claimed during his campaign that neighbors saw explosives at the home of the attackers but neglected to alert law enforcement. Others have also suggested neighbors didn't report activities at the home out of some sense of political correctness because the couple was Muslim. But there is no evidence that witnesses saw weapons in the Redlands townhouse.
Levin said it's human nature to suspect that such a deadly massacre had to be the work of more than just two people.
"The fact the loner or duo can operate autonomously and do such damage is all the more concerning to people and leads people to search for greater explanation when gaps in information exist," he said. But "operationally everything we know suggests no one else is involved."
Farook left his Redlands home at 8:37 a.m. on Dec. 2 and arrived at the Inland Regional Center 10 minutes later. His co-workers were gathered for a holiday event and training session there. He carried with him a bag containing a bomb, authorities say. He stuck around the center for about two hours before leaving. He returned to the event at 10:56 a.m. with his wife. They were armed with AR-15s and their faces were covered. They sprayed the room with bullets before fleeing. They left behind an explosive device made of several pipe bombs.
From there, it appears that the couple Zigzagged around greater San Bernardino. Traffic cameras, surveillance footage, witnesses and cell phone towers gave FBI agents glimpses of their seemingly haphazard route.
But the FBI cannot account for their whereabouts between 12:59 and 1:17 p.m. When the couple resurfaced, they were about a mile from the regional center, which by that time was surrounded by police and federal agents. Investigators believe they might have been returning to the scene in an attempt to remotely detonate a bomb they left behind, possibly in a bid to kill first-responders. No bomb went off.
FBI officials have said the 18-minute gap is a crucial part of the investigation because it's possible the couple met with someone during that time, though investigators can only speculate.
"Until we know what happened in those 18 minutes, I am uncomfortable, and my investigators are uncomfortable," FBI assistant director David Bowdich said earlier this year.
The FBI said it still has limited evidence of how the couple planned the attack or information about why they targeted Farook's colleagues. Investigators had hoped the couple's electronic equipment could provide answers. But that's been another struggle.
Agents found that the hard drive of Farook's home computer was missing.
In an effort to find the hard drive, FBI agents scoured the bottom of Seccombe Lake, in the heart of San Bernardino. The couple had pulled over briefly in a parking lot near the lake at 11:45 a.m. FBI agents now theorize that the hard drive might have been thrown into a trash bin and be buried in a landfill.
A second cellphone Farook carried was so badly broken by him that investigators could not extract any data. The unlocking of the iPhone 5c that set off a national furor over security and privacy revealed only work-related data from his job as a health inspector, Eimiller said.
"The goal was to exploit all available digital evidence in this case in the event it contained data of investigative value," Eimiller said.
Sources say the phone, which was owned by San Bernardino County, yielded limited information. It mostly had schedules for Farook's health inspectors.
Authorities still aren't exactly sure why the couple made the Christmas party the target of their attack. San Bernardino Police Lt. Mike Madden noted that in 2014, Farook attended another work event in that room, which was decorated for Christmas. Malik later stated on an online account something to the effect that she felt Muslims should not have to attend Christian events, Madden said.
Madden added that she did not specifically mention the event in her posting and that it's far from clear that it played any role in the attack.
"It is open to speculation why they chose the party," he said. "I don't know if we can say with any certainty why the party was chosen apart from it was a soft target."
San Bernardino Police Chief Jarrod Burguan agreed.
"There are a lot of questions and we don't have all the answers," he said Thursday. "We never established the motive. The best we can do is theorize."
Experts said it could take years to better understand how the San Bernardino plot was hatched and carried out. And even then, there will likely be many nagging, unanswered questions.
"The scope of what the San Bernardino couple prepared for is a reason alone for investigators to continue digging," said Erroll Southers, a former presidential nominee for assistant secretary of the Transportation Security Administrationand a USC professor of homeland security. "In terrorism investigations there can be frustrations. At this point they may have exhausted the electronic trail. But it doesn't mean they are going to stop."
2:30 p.m.: This story was updated with comments from the police chief.
11 a.m.: This story was updated with details from Police Lt. Mike Madden.