NEW YORK — Rescue workers using dogs and thermal-detection gear to search the rubble Thursday for more victims of a gas explosion found an eighth body, while investigators tried to pinpoint the leak and determine whether it had anything to do with New York’s aging gas and water mains, some from the 1800s.

At least five people were unaccounted for after the deafening blast Wednesday morning destroyed two five-story apartment buildings in East Harlem that were served by an 1887 cast-iron gas main. More than 60 people were injured.

Fire and utility officials said that if the buildings were plagued in recent days or weeks by strong gas odors, as some tenants claimed, they have no evidence anyone reported it before Wednesday.

Robert Sumwalt of the National Transportation Safety Board said the gas main and distribution pipe under the street had been examined in a crater and were found to be intact, with no obvious punctures or ruptures. They had not been torn from the ground, he said.

However, he said NTSB investigators had been unable to conduct a more thorough examination because of the rescue effort underway, and it was still unclear whether the leak came from inside or outside the buildings.

He said there had also been a water main break at the site, but it was not known if that contributed to the gas explosion or was caused by it. The water main was installed in 1897, according to the city.

Authorities also hoped to reach the basement — still under rubble — to examine heating units, meters and other equipment that might hold clues to the blast, Fire Commissioner Salvatore Cassano said.

“We can only get conclusive evidence when the fire is out, when the rescue is completed, and we really get a chance to look at all the facts,” Mayor Bill de Blasio said.

Aging infrastructure — crumbling bridges, highways, water mains and gas lines — has become a major concern in recent years, especially in older cities in the Northeast, and has been blamed for explosions, floods and other accidents.

Officials said workers were about 40 percent to 50 percent through the rubble, using sound devices and putting telescopic video cameras into small voids to see if there is someone in there.

The mayor told firefighters carrying grappling hooks and other equipment: “I can only imagine, knowing that at any moment you might find a body, how difficult that is.”

Police identified four of the dead: Griselde Camacho, 45, a Hunter College security officer; Carmen Tanco, 67, a dental hygienist who took part in church-sponsored medical missions to Africa and the Caribbean; Andreas Panagopoulos, a musician; and Rosaura Hernandez, 22, a restaurant cook from Mexico.

Mexican officials said a Mexican woman, Rosaura Barrios Vazquez, 43, was among those killed.

The bodies of two unidentified men were also pulled from the rubble.

At least three of the injured were children. One, a 15-year-old boy, was reported in critical condition Thursday with burns, broken bones and internal injuries. A woman was in critical condition with a head injury.

The blast erupted about 15 minutes after someone from a neighboring building reported smelling gas, authorities said. Con Ed said it immediately sent workers to check out the report, but they got there too late.

Con Ed CEO John McAvoy said the call had been correctly categorized as low priority. 

“A single person calling that they smelled gas outside of a building is not something that would warrant a fire department response,” he said.

After the disaster, a number of neighborhood residents said they smelled gas on Tuesday but didn’t report it. A tenant in one of the destroyed buildings, Ruben Borrero, said that residents had complained to the landlord about the gas odors on Tuesday and that fire officials were also called a few weeks ago.

But Cassano and McAvoy said that before Wednesday, the fire department and Con Ed had received no complaints in the last 30 days about a gas leak in the area.

An Associated Press analysis of the city’s 311 calls database between Jan. 1, 2013, through March 11 also found no calls from the buildings about gas.

The lesson, De Blasio said, is that because of the city’s old and vulnerable infrastructure, people should heed the post-9/11 slogan “If you see something, say something.”

“Every citizen of this city should certainly always tell the city government if they see a problem,” he said.

Jonathan Lemire, Associated Press