ALLENTOWN — The search for five victims of a massive explosion that rocked a Pennsylvania neighborhood has reached its grim conclusion, but the search for clues about what caused the disaster that leveled two houses and burned many more has only begun.

Investigators and utility workers picked through the smoldering ruins of a rowhouse neighborhood Thursday, trying to determine whether an 83-year-old cast-iron gas main was the culprit in a thunderous explosion that killed five people.

The fiery blast late Wednesday night was the latest deadly natural-gas disaster in recent months to raise questions about the safety of the nation’s aging, 2.5-million-mile network of gas and liquid pipelines.

The explosion, which flattened a pair of rowhouses and set fire to a block of homes, occurred in an area where the underground gas main lacked shut-off valves. It took utility workers five hours of toil in the freezing cold to punch through ice, asphalt and concrete and seal the 12-inch main with foam, finally cutting off the flow of gas that fed the raging flames.

Dorothy Yanett, 65, said she was in her living room with her husband awaiting the evening news when she heard a series of booms.

“Everything falling and crashing, glass, just a nightmare,” she said. She found glass in the shoes she was going to put to leave the house. “There was no odor, there was no smell. Then it was like all hell broke loose.”

Joe Swope, a spokesman for Reading-based UGI Utilities Inc., said that a routine leak-detection test in that area had come up clean on Tuesday, and that there had been no calls about gas odors before the disaster.

Lehigh County Coroner Scott Grim said Thursday afternoon that four bodies had been recovered — a 4-month-old boy, a 16-year-old girl, a 69-year-old woman and a 79-year-old man. Cadaver dogs found the fifth victim, a 74-year-old woman, in the rubble on Thursday night, Fire Marshal Matthew Bainbridge said. Their names were not immediately released.

Forty-seven homes were damaged; eight of them appeared to be a total loss, said Allentown Fire Chief Robert Scheirer. The exact spot of the explosion and what triggered it were under investigation.

“The investigation will look at the 12-inch main, but will also look at service lines that feed gas into the nearby homes and businesses, as well as potential causes inside the home,” Scheirer said. “Until that investigation is complete, it’s premature to conclude exactly where the leak took place.”

Investigators planned to send cameras through the main to look for cracks, and perform air pressure tests on the service lines.