He said the couple had been married for about a year and had recently been through what he described as "a very contentious divorce."
Police said they do not know how many of the victims died of gunshot wounds or from the subsequent fire.
According to police, Pardo, who was carrying at least two handguns and a home-made incendiary device, began shooting as soon as he entered the house.
"It looks like it was planned," Pat Buchannan, a police spokesman, told the Reuters.
"It looks like he arrived with at least two handguns, and this device that we have described that somehow propelled a flammable liquid throughout the downstairs area of the residence."
Witnesses among the 25 people attending the Christmas party told police that Pardo stripped off his Santa costume after the shooting and fled in street clothes.