As he had in years past, Bruce Jeffrey Pardo volunteered to serve as an usher at his church’s midnight Mass on Christmas Eve.

Pardo, however, was miles away from the Holy Redeemer Catholic Church as the religious service got underway. He had driven a rental car from his home in Montrose to the Covina home of his ex-wife’s parents. Shortly before 11:30 p.m., dressed as Santa Claus, he approached the front door with a large, wrapped package.

Inside the two-story home at the end of a quiet cul-de-sac, his ex-wife, her parents and about two dozen others were enjoying their annual holiday party.

An 8-year-old girl ran to answer his knock. When the door swung open, Pardo, 45, shot her in the face with a semiautomatic handgun, stepped into the house and opened fire on the revelers. Amid the chaos, he doused the house with a flammable liquid contained in the package -- a pressurized fuel tank, about 2 1/2 feet tall.

Partygoers fled in panic as the house on Knollcrest Drive went up in flames. They ran to neighbors’ homes and frantically called 911. A young woman, according to a neighbor, leaped out of a second-floor window, breaking her ankle.

The 8-year-old girl and a 16-year-old girl who was shot in the back survived and were transported to local hospitals with non-life-threatening injuries, officials said. Eight bodies remained in the ruins of the house, coroner officials later said.

It was thought to be the worst single killing spree in the county this year. Overall, homicides have remained at relatively low levels compared to previous years.

The investigation, which began quickly and continued into Christmas Day, led to two other Southern California homes: Pardo’s and his brother’s, where Pardo was found dead. It revealed a divorce, the flammable device like the one Pardo took into the Covina home -- police said they’d never seen anything like it -- and incredulous reactions from people who knew Pardo as a reliable church usher and good, but quiet, neighbor. By late Thursday night, authorities weren’t even close to identifying all the victims, although his former wife and her parents are thought to have been killed.

In Covina, police kept firefighters at bay as flames engulfed the house, believing the gunman might still be in the area. About 80 firefighters battled the blaze for an hour and a half before extinguishing it, said Los Angeles County Fire Capt. Mike Brown. The second floor of the house fell to the ground. Palm trees swayed eerily in the smoke.

Pardo had left the scene quickly, driving to his brother’s house about 40 miles away in Sylmar. Shortly before 3:30 a.m., the brother summoned Los Angeles police, saying he had come home to find Pardo dead with a gunshot wound to the head. When arriving officers ran Pardo’s name through a law-enforcement database, they were alerted that he was wanted by Covina police.

By 9 a.m., a pair of Covina detectives had arrived at Pardo’s tan stucco house in Montrose and cordoned it off with tape. A wreath was hung on the front door and candy canes adorned the fence.

An SUV and a military-style Hummer were parked in the driveway.

In midafternoon, authorities approached the house with guns drawn, shouting, “We’re police! We have a search warrant!” When no one responded, they used a battering ram to get in. More than two hours later, officers carried away four shotguns, a handgun, wrapping paper and a fuel tank like the one Pardo used in the attack.

A label on the red tank read, “WeFuelFun.com,” the website of a company that specializes in fueling devices for all-terrain vehicles. Buchanan, a 30-year police veteran, said the tank Pardo used in the attack was “nothing that we or the arson team had ever seen.”

Recently, Pardo had been living in the Montrose home alone, said Det. Antonio Zavala. Court records show that his wife divorced him last September. He “was apparently going through a bad time in his marriage,” Covina Police Lt. Pat Buchanan said.

Neighbors said that Pardo had lived in the house with his wife, Sylvia, 43, and her three children for a few years, until she and the children moved out last spring.

The family was often seen walking their dog in the neighborhood.

Pardo, several neighbors said, was a quiet, unassuming man who enjoyed tending his garden and regularly ushered at the evening Sunday Mass at Holy Redeemer.

“Bruce?” said an incredulous Jan Detanna, the head usher at the church, when told about the attack by a reporter on the phone. “I’m just -- this is shocking. He was the nicest guy you could imagine. Always a pleasure to talk to, always a big smile.”

Bong Garcia, Pardo’s next-door neighbor, said he last saw Pardo between 9 and 10 p.m. Christmas Eve. Pardo, who was dressed in regular clothes, said he was on his way to a party.

Pardo’s former wife and her parents, Joseph Ortega, 80, and Alicia Ortega, 70, are believed to be among the dead. Authorities said Thursday afternoon that the bodies found in the house were so badly burned that dental records would be needed to identify them.

The Ortegas lived in a bucolic neighborhood -- their street lined with large homes with swimming pools, valued upward of half a million dollars.

Tom Minter, 78, who lives a few doors down from the Ortegas’ house, was washing dishes when he heard a loud bang. Soon after, police SWAT officers rushed a man and two women who had been at the party into his home, sat them down in the hallway and turned off all lights in the house, Minter said. Both women were crying and the man was talking into his cellphone, saying ‘They’re all dead, he shot them all,’ ” Minter recalled.

Gazing at the scene of destruction on Christmas morning, Frank Castillo, 46, stood at the yellow police tape barrier trying unsuccessfully to obtain information about relatives he said had been at the party.

“I want to make sure [they] are OK,” he said, choking back tears.

The bizarre case continued to unfold into the night. Bright lights brought in by police illuminated the crime scene in Covina. In searching the rental car in Sylmar, the L.A. County sheriff’s bomb squad found a suspicious object.

As they attempted to render the object safe, the car erupted in flames, said Los Angeles Police Sgt. James Bender. Later, police evacuated houses close to Pardo’s in Montrose, concerned that he might have left behind bombs.

ari.bloomekatz@latimes.com

tami.abdollah@latimes.com

joel.rubin@latimes.com

Times staff writers Raja Abdulrahim, Larry Gordon, Victoria Kim and Alexandra Zavis contributed to this report.