 -- An early morning house explosion in Elizabeth, N.J., today that left at least one person dead and at least 15 injured, including an 11-year-old, is being described as a "gas episode" by authorities.

The blast occurred around 8 a.m. on the second floor of a two-family duplex on Magnolia Avenue.

Elizabeth Mayor Chris Bollwage said the explosion was definitely a "gas episode," according to ABC station WABC. He also said that when first responders arrived on the scene, residents said they'd smelled gas.

Bollwage said an investigation into the explosion continued.

A fire that eventually self-extinguished left two victims severely burned, including the 11-year-old, Bollwage said. Those victims were being treated at St. Barnabas Hospital's burn unit, Bollwage said.

He said that a person who'd originally been reported as missing had been found safe with no injuries.

The remaining victims, whom Bollwage said were mostly sleeping in the rear of the building at the time of the explosion, were being treated at other area hospitals. None of the victims had been identified yet but Bollwage said that it appeared all of the victims were inside the two-family duplex that exploded.

Wayne Sheppard, who was sleeping in a second-floor apartment across the street from the house, said he woke up in a "nightmare." His wife and 10-month-old baby were also in the apartment at the time, he said.

"She had just taken him out of his bedroom," Sheppard told WABC. "His bedroom window is directly across from the house that exploded."

Authorities said the home was legal, up to code and had been built in the last eight years.

"We don't know if any current work was being done at the time," Bollwage said. "It was a relatively new construction but we are not aware of any current work."

At least two buildings would be demolished as well as a possible third, authorities said, noting that all of the buildings in the vicinity of the blast -- a total of about seven -- would be inspected.

Elizabeth Fire Chief Thomas McNamara said that the explosion had "pancaked" the second floor onto the first floor.

"Where the explosion came from, we are not sure," McNamara said, adding that it's going to take "quite a while to make sure the area is safe."