The terror attack that left 14 people dead one year ago in San Bernardino, Calif., my have been triggered by a training session and lunch with holiday decorations that the shooter was forced to attend, authorities said.

Syed Farook opened fire on fellow county workers at the Inland Regional Center on Dec. 2, 2015. 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 attend.

Just before the attack, Farook had posed with four fellow county employees in front of a Christmas tree in the conference room.

A short time later, 14 people were killed and 22 others injured when Farook and his wife 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.

Burguan said that is one of the few pieces of evidence they can point to as a probable motive in the case.

Farook and his wife died in a shootout with police.