San Bernardino Massacre Triggered by Holiday Party Dispute

One of the gunmen who shot up the Inland Regional Center in San Bernardino was at a holiday party at the facility earlier Wednesday ... got into a dispute, then came back with his "buddies" and opened fire ... this according to a well-placed law enforcement source.

An LAPD source who has been well-briefed tells us, one of the gunman showed up for some sort of Christmas party for employees of the San Bernardino County Health Dept. The party was being held in one of the conference rooms that was rented out for the event ... and when the gunman returned, he went to same room and shot it up.

We do not know the nature of the dispute or why the gunman was at the party, but we're told the man was either kicked out or left voluntarily ... then came back with his friends.