After blasts injure five at Oktoberfest in Huntington Beach, many ask: What caused them?

The fire crew arrived just after 8 p.m. Saturday at the Old World Village’s German Restaurant, where an Oktoberfest celebration was in full swing. The restaurant owner, Bernie Bischof, led two firefighters to a patio area, which he’d cordoned off from patrons a few minutes earlier after smelling a strange odor emanating from an underground vault that held three transformers.

A week earlier, he’d heard unfamiliar noises coming from the vault.

As Bischof, two of his staff members and the firefighters approached the vault, an explosion tore off its 50-pound door and knocked them to the ground, said Jeff Lopez, a battalion chief for the Huntington Beach Fire Department.

“Our firefighters had to literally pick themselves up off the ground and figure out what was going on,” Lopez said Sunday morning. The explosion had thrown a burning material — believed to be mineral oil from inside the transformers — into the air, burning the firefighters, Bischof, a server and a security guard.

They scarcely had time to get up and realize what was happening when a second explosion rocked the Bavarian-style eatery “a minute or so later,” Lopez said.

The blasts shattered the beery euphoria of Oktoberfest and left many on Sunday wondering what caused them. It was the busiest weekend of the year for Bischof’s restaurant, which at the time of the explosion was filled with 1,000 people, or two-thirds the night’s expected turnout, said Dan Escamilla, a member of the restaurant’s management team who was tapped as its spokesman after the explosions.

Josef Bischof — Bernie Bischof’s father — came to the United States from Germany in 1952 and developed the enclave of Bavarian-style restaurants, shops and bars in Huntington Beach known as the Old World Village in the late 1970s, according to the village’s website and a 1986 Times article. Its inaugural Oktoberfest celebration was held in 1978.

Of the two firefighters and three civilians who were hospitalized Saturday night, all but one civilian had been released by Sunday, Lopez said. He declined to name the hospitalized civilian, citing privacy concerns. But Escamilla said the person was Bischof, who he said was in critical condition on Sunday at UCI Medical Center, having suffered second- and third-degree burns on 38% of his body.

After the explosions, Bischof ran out of the patio and into the restaurant, where two security guards wrapped him in a tablecloth to smother the flaming oil that clung to his arms, legs, flank and head, Escamilla said. The restaurant suffered no structural damage, and the fire in the patio area was extinguished within 10 minutes, he said.

Southern California Edison has replaced the three transformers that were within the restaurant’s vault, according to Robert Villegas, a company spokesman. An Edison crew initially had to wait on Saturday night to enter the vault because police were concerned the explosions may have been caused by criminal activity, Villegas said.

Those concerns weren’t borne out, however, and Edison was able to replace the transformers overnight and restore power by 9 a.m. Sunday to some 300 customers who were left without it overnight, Villegas said. About 1,700 Edison customers in all lost power for several hours after the explosions.

Edison has cataloged the damaged equipment and is investigating the explosions’ cause, which hasn’t been determined, Villegas said.

One of the transformers beneath the restaurant exploded in June 2010, according to Escamilla; the blast occurred early in the morning and no one was injured, he said. The transformers were making “strange noises” as recently as a week ago, which Bischof had asked Edison to look into, Escamilla said.

“Edison sent someone to check and they signed off on it,” he said. “Edison dropped the ball on this, simply put.”

Villegas, the Edison spokesman, said the company is still gathering maintenance records for the transformers and had no immediate comment.

Lopez, the fire chief, said the strange odor that prompted the restaurant’s staff to call the fire department was probably mineral oil overheating inside the transformers. He credited the staff for clearing the area of patrons, which he said limited the number of injuries to a handful.

“On a Saturday night, especially an Oktoberfest night, there’s a lot of people in that restaurant,” Lopez said. “It could’ve been a lot worse.”

Three and a half hours before the transformers exploded, Escamilla said, a family was celebrating a 1-year-old’s birthday at a long table “2 feet from the vault.”

“I have no doubt that if Bernie hadn’t cleared the area when he noticed the smell, people would’ve lost their lives,” Escamilla said. “There would have been 30 to 40 people in that area if he had not done that.”

He described the evacuation of patrons as “fast and orderly,” in large part because there had been an active shooter threat directed on Thursday at a movie theater screening “Joker” a few blocks from the Old World Village.

“We were already on high alert,” Escamilla said.