San Bernardino Attack Possibly Triggered by Christmas Party, Police Say

Exclusive photos show shooter posing in front of a Christmas tree with colleagues before the attack.
2:34 | 12/01/16

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Transcript for San Bernardino Attack Possibly Triggered by Christmas Party, Police Say
Next tonight, the ABC news exclusive. New images and video from one of the deadliest terror attacks in American history. In San Bernardino, one year ago tomorrow. The husband killing 14. Tonight, right here the new image taken in the moments before they opened fire. The terrorist standing right beside his co-workers who thought he was a friend. Here's ABC's Brian Ross. Reporter: It was an annual tradition with the San Bernardino health department, a meeting and then a holiday party. And tonight officials tell ABC news, new evidence shows the Christmas decorations, the tree, the party may have been the trigger that led this employee, Syed Farook, and his wife to go on their terror rampage. 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. Reporter: San Bernardino police chief Jarrod burguan, tells ABC news, the shooting began shortly after Farook posed in front of the Christmas tree and then left the room, agitated and upset. That possibly is a motive in this case. Reporter: Officials now say Farook, an American citizen, and his wife, Tashfeen Malik, who came here from Pakistan on a fiancee Visa, were sharply focused on the holiday gathering. This surveillance video obtained by ABC news, shows Farook at a California firing range just two days before the attack, practicing with the weapons he and his wife would use. 911. We have an active shooter. Ma'am, listen to me, take a deep breath. Okay, is he still shooting? Oh, my god. Reporter: Farook and his wife killed 14 of his fellow workers at the party, and wounded 22 more. How many rounds did he fire? Oh my god, a million. Reporter: A short time later they posted a message on Facebook pledging allegiance to ISIS leader Abu Bakr al-baghdadi. But officials say the couple actually had to conduct an online search that very morning, who is the leader of ISIS. Maybe they didn't truly know who it was. Reporter: Two lone wolves, who died in a fierce gun battle with police, after killing so many. Just incredible that was a year ago tomorrow. Brian here with us tonight. They had more terror planned that day. That's right, David, according to authorities they left behind three pipe bombs in that conference room and before they were stopped by police, they were circling the neighborhood to detonate those bombs and take more lives. Still much more ahead on

This transcript has been automatically generated and may not be 100% accurate.

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