Bruce Jeffrey Pardo, 45, a devoted Roman Catholic and aerospace industry worker who may have recently lost his job, shot himself in the head hours after the meticulously planned carnage in Covina, California, 22 miles east of Los Angeles.
His murderous rampage began late on Christmas Eve when an eight year-old girl opened the two-storey house’s front door in the quiet cul-de-sac to find Pardo, dressed as Father Christmas and carrying a large present.
When the door opened, Pardo fired the handgun at the eight-year-old daughter of Leticia Yuzefpolsky, a sister of Sylvia Pardo, as she ran to greet him, injuring her in the face.
He then fired indiscriminately at fleeing party-goers.
A man who dressed up as Santa Claus to slaughter nine people at a Christmas party used a home-made flamethrower as 'a Christmas present' to set his house on fire.
Witnesses saw Bruce Pardo use a metal device disguised with a Christmas bow to spray high-octane car-racing fuel around the home.
Police speculate that Pardo may have stood over and pointedly executed some of the victims, using the other handguns.
Nine people died from either gunfire or flames, and three others were wounded: the eight-year-old girl who was shot in the face with severe but non-life-threatening injuries, a 16-year-old girl shot and wounded in the back, and a 20-year-old woman who suffered a broken ankle jumping out of the second-floor window.
His car, parked nearby and containing ammunition, exploded on Christmas Day after he rigged it with explosives.
None of them could be positively identified until dental records are examined.
It was unclear whether the gunfire, flames or smoke had killed them.
He worked at the Jet Propulsion Laboratory in La Cañada Flintridge.
He met Sylvia Pardo, previously Sylvia Orza, in 2004. PST, Bruce Jeffrey Pardo, dressed in a Santa Claus suit, knocked on the door of his former in-laws' house, occupied by about 25 people, with a gift-wrapped package containing a homemade flamethrower in one hand and a 9mm semi-automatic handgun in the other hand; he also had three additional 9mm semi-automatic handguns in his possession.