Authorities believe the terrorist attack on Dec. 2, 2015, in San Bernardino may have been triggered by a mandatory employee training session and lunch replete with holiday decorations, including a Christmas tree, that shooter Syed Farook was forced to attend.
Emails discovered by the FBI and police reveal Farook's wife, Pakistani native Tashfeen Malik, objected to the Christmas setting and was upset her husband had to go.
Just before the attack, Farook posed with four fellow county employees in front of a Christmas tree in a conference room.
A short time later, 14 people were killed and 22 others injured when Farook and Malik opened fire at the meeting with assault rifles.
"She had essentially made the statement in an online account that she didn't think that a Muslim should have to participate in a non-Muslim holiday or event," said San Bernardino Police Chief Jarrod Burguan in an exclusive interview with ABC News that aired on ABC News' "Nightline."
"That really is one of the very, very few pieces of potential evidence that we have that we can truly point to and say, 'That probably is a motive in this case,'" he said.
Four hours after their rampage, Farook and Malik were shot dead in a gunbattle on a San Bernardino street with police.
Farook at the RangeNew video, obtained by ABC News, shows Farook two days before the attack practicing at a local firing range with a pistol and an assault rifle that a friend bought for him.
The video shows Farook adjusting the sights on his rifle and then firing at paper torso silhouette targets, one of which was later recovered in the shooters' vehicle and led authorities to the range.
"They had high-powered weaponry. They had lots of ammunition. They had bombs at their disposal," said Burguan.
An analysis by the FBI shows that after the two shooters left the scene of the shooting, they never went more than 7 miles from the site and were constantly circling, often driving right past police barricades.
Officials now believe they were trying to remotely set off three pipe bombs they left behind that failed to detonate.
The remote detonator was discovered in their car after the final shootout.
An Ignorant ISIS PledgeShortly after the attack, the couple posted an online message pledging their allegiance to the ISIS terrorist group and its leader, Abu Bakr al-Baghdadi.
But police say the couple had to search online earlier in the day for the name of al-Baghdadi.
"You would think that somebody that truly wanted to do that, somebody that was truly committed, certainly wouldn't be researching on that particular day," said Burguan.
The murderous couple left behind a 6-month-old daughter, who remains in state foster care, according to family members.
"It certainly surprised me that you'd have a woman with a 6-month-old baby who'd do something like this," said David Bowdich, who was the in charge of the FBI Los Angeles office. "That's very hard for us to comprehend."
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