By the time Ned Mihaylov rushed outside early Wednesday, all that remained of his neighbor’s house was a pile of burning rubble.

The brick home with a stone base in Montgomery County had been leveled by a natural gas explosion, and now the hunt was on for the two people who were apparently thrown from the home by the blast, Mihaylov and authorities said.

Mihaylov, 26, who lives next door to the destroyed house in the 11200 block of Ashley Drive, said he heard a man calling for assistance, then spotted a woman in pajamas lying next to the wreckage. He said he jumped a fence to get to her and covered her face from the pouring rain.

“She was just saying, ‘Help me. Help me. I can’t breathe,’ ” Mihaylov said. “Her laying out on the ground, I was just really scared.”

The man and the woman, both in their 30s, were taken to a hospital with serious injuries, authorities said. The man is expected to survive; the woman’s condition was less certain, authorities said.

Fire officials blamed the 3 a.m. blast on the male occupant’s attempt to replace a gas-powered dryer in favor of an electric model before going to bed. They said something he did in that process allowed gas to seep into the home, and when the gas hit a spark, the home exploded. The blast is estimated to have caused about $750,000 in damage, fire officials said.

The blowup could be heard miles away, neighbors said, and woke up nearly everyone in the close-knit community of Randolph Hills, an area near Garrett Park, southeast of downtown Rockville. Many residents rushed to the scene.

“It was crazy,” said Matthew Tifford, president of the Randolph Civic Foundation. “Everyone was kind of coming out their front doors.”

By Wednesday afternoon, all that was left of the home was a massive pile of cinder blocks and wood. The largest intact piece of the structure appeared to be a door that had been catapulted about 100 feet straight back into a tree. Debris littered the yards of adjacent homes.

Eileen White, a neighbor, said the explosion jolted her awake, and when she ran outside, “the house was gone.”

“It was just sort of disbelief,” White said. “You don’t expect to come out and see your neighbor’s house just completely gone.”

Most neighbors said they did not know that anyone was living in the house, which had been vacant for months. Mihaylov said he had only met the couple when they moved in a few days earlier. Efforts to identify and reach them and the homeowner were unsuccessful.

Mihaylov said the man’s legs were bleeding, but he was upright and walking, searching through the rubble for the woman. He said the woman “looked hurt” and was talking to him in a soft voice as he tended to her. “I just told her, ‘Stay calm,’ ” Mihaylov said.

The explosion is the second in the Washington area in recent days. On Monday morning, a home in the 19000 block of Wayne Drive in Dumfries was leveled by a similar blast , which Prince William County fire officials preliminarily blamed on a gas leak. The fire department issued a news release Wednesday saying that firefighters on the scene smelled gas, but the exact cause was still under investigation. According to the release, the blaze caused about $130,000 in damage.

Staff writer Justin Jouvenal and staff researcher Magda Jean-Louis contributed to this report.