The last picture of Syed Farook shows him posing at his office Christmas party with colleagues before he killed 14 of them in San Bernardino.
US ABC News' investigative team discovered emails between Farook and his Pakistani wife, Tashfeen Malik, which indicate she was upset her Muslim husband was being forced to attend a Christmas gathering.
Not long after Farook was pictured in front of this Christmas tree, he killed 14 colleagues and injured 22 others.
"[Malik] 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," San Bernardino police chief Jarrod Burguan told ABC News.
"That really is one over 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'."
Four hours after their rampage, Farook and Malik were shot dead by police.
ABC News also reported on new video footage, which shows Farook at a firing range two days before the attack.
An FBI investigation also found that Farook and Malik continued to drive around the same area while being pursued by police so they could remotely set off three pipe bombs that failed to detonate. The detonator was found in the car after they were killed.
Before they were killed, the couple posted an online message pledging their allegiance to Isis and its leader, Abu Bakr al-Baghdadi.
However the FBI says Google searches indicate that the couple had to search for al-Baghdadi's name a day earlier.
"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.
He said he found it hard to comprehend that a mother with a 6-month-old baby would leave the child behind. The daughter is now in foster care.