ABC News learned it may have been Tashfeen Malik's anger over her Muslim husband, Syed Farook, having to attend a work event that included Christmas festivities that triggered the deadly violence on Dec. 2, 2015.
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The terrorist couple opened fire inside the Inland Regional Center during a holiday party, killing 14 people and injuring 22 others.
Newly released photos show Farook posing with his coworkers inside the conference room that day not long before the attack happened.
In a chilling video, the couple is shown preparing to carry out their violent plan by practicing at a shooting range just two days before. Farook used his pistol and assault rifle in the video.
First responders recalled rushing in to save people inside the center and stumbling upon a horrific scene.
"There was this river of blood coming out of the conference room onto the sidewalk, onto the walking pavement area," said Nathan Scarano, a San Bernardino County probation officer.
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Victims were rushing out as rescuers ran in. Scarano recalls hearing a faint voice calling for help. It was Amanda Gaspard, 32, who was near death having lost half her blood after the couple shot her.
"I was begging, 'Please help me, please help me,'" Gaspard said. "People were moaning all around me and I said, 'Please come here, help me.' Dead bodies all around."
Nearly a year later, Gaspard is still in pain and recovering. She also had a message for Scarano, who saved her life, as well as the other first responders.
"You are one of God's angels here on Earth. I believe I have guardian angels, but that day God used all those first responders to get in there and to get us out so that we are still alive today," she said.
On Friday, the city will be holding a night of remembrance memorial at Cal State San Bernardino to mark the first anniversary.