The husband and wife shooters who massacred 14 people in San Bernardino were at least partly motivated by objections to a mandatory Christmas-themed work event, police have said.

While officers still don’t know exactly what led Syed Farook and his wife Tashfeen Malik to open fire on a conference room full of people, new details have emerged of how events unfolded one year ago on 2 December 2015.

According to a new investigation by ABC News, emails were discovered by the FBI which showed Malik was upset her husband had no choice but to attend a training session and lunch at the Inland Regional Centre (IRC) in San Bernardino.

Pictures taken at the event just minutes prior to the shooting show Farook posing with four fellow county employees wearing festive clothing, and all of them stood in front of a Christmas tree.

San Bernardino police chief Jarrod Burguan told ABC News of Malik: “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.

“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.’”

The shooters were on the run for four hours after their rampage, in which 22 others were injured, most severely, and during that time they uploaded an online message declaring their allegiance to the Isis militant group.

But Burguan also told ABC News that police now know the couple had to google the name of the leader of Isis, Abu Bakr al-Baghdadi, earlier in the day.

“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,” he said.

Farook and Malik were eventually found to be driving a black SUV not far from the scene, after its licence plate was noted by a member of the public, and both were killed in a shootout with police.

Three unexploded pipe bombs were later found at the scene of the shooting, and officials now believe the couple returned to the IRC conference centre and kept trying to get close to it in a bid to use a remote detonator and set them off.