NEW YORK (AP) ― A gas leak triggered an earthshaking explosion that flattened two apartment buildings on Wednesday, killing at least six people, injuring more than 60 and leaving four others missing. A tenant said residents had complained repeatedly in recent weeks about “unbearable” gas smells.

By evening, rescue workers finally began the search for victims amid the broken bricks, splintered wood and mangled metal after firefighters spent most of the day dousing the flames. Heavy equipment, including back hoes and a bulldozer, arrived to clear the mountain of debris where the two five-story East Harlem buildings stood. Floodlights were in place. Thermal imaging cameras were at the ready to identify heat spots ― bodies or pockets of fire.

The recovery was made difficult by of the weather, which was expected to drop below freezing with rain. Some parts of the debris pile were inaccessible because of a sinkhole caused by a subsurface water main break, officials said.

The fiery blast, on Park Avenue at 116th Street, not far from the edge of Central Park, erupted about 9:30 a.m., around 15 minutes after a neighboring resident reported smelling gas, authorities said. The Con Edison utility said it immediately sent workers to check out the report, but they did not arrive until it was too late.

The explosion shattered windows a block away, rained debris onto elevated commuter railroad tracks close by, cast a plume of smoke over the skyline and sent people running into the streets.

“It felt like an earthquake had rattled my whole building,” said Waldemar Infante, a porter who was working in a basement nearby. “There were glass shards everywhere on the ground, and all the stores had their windows blown out.”

Emanuel Rivera, 24, lives on the corner of 116th and Park Avenue, across the street from the blast site. He was at home sleeping with his wife Rehanna and their two children when it happened.

“All you heard was boom. The whole building shook. We looked outside and there was a bunch of smoke. Everything was in flames,” he said.

Like several other neighbors, Rivera said they started smelling gas Tuesday night.

“The smell of gas was there since yesterday night,” he said.

Hunter College identified one of the people killed in the blast as Griselde Camacho, a security officer who worked for the college since 2008.

Another of the people who died was Carmen Tanco, 67, a dental hygienist.

Police identified the third fatal victim as Rosaura Hernandez-Barrios, 21.

The body of an adult male was pulled from the rubble just after midnight Wednesday, according to FDNY spokesman Jim Long. He said the man was pronounced dead at the scene.

Authorities said that a body of an adult female and a male were found among the debris early Thursday.

At least three of the injured were children; one, a 15-year-old boy, was reported in critical condition with burns, broken bones and internal injuries. Most of the other victims’ injuries were minor and included cuts and scrapes.

A tenant in one of the destroyed buildings, Ruben Borrero, said residents had complained to the landlord about smelling gas as recently as Tuesday.

A few weeks ago, Borrero said, city fire officials were called about the odor, which he said was so bad that a tenant on the top floor broke open the door to the roof for ventilation.

“It was unbearable,” said Borrero, who lived in a second-floor apartment with his mother and sister, who were away at the time of the explosion. “You walk in the front door and you want to turn around and walk directly out.”

The fire department said a check of its records found no instances in the past month in which tenants of the two buildings reported gas odors or leaks.

Edward Foppiano, a Con Ed senior vice president, said the block was last checked on Feb. 28 as part of a regular leak survey, and no problems were detected.

City records show that the building Borrero lived in was owned by Kaoru Muramatsu. A phone number listed for Muramatsu rang unanswered.

Con Ed said it remains to be seen whether the leak was in a company main or in customer-installed inside plumbing. The gas main that serves the area was made of plastic and cast iron, and the iron dated to 1887, Foppiano said.

A National Transportation Safety Board team arrived in the evening to investigate. The agency investigates pipeline accidents in addition to transportation disasters.

Just before the explosion, a resident from a building next to the two that were destroyed reported smelling gas inside his apartment and thought the odor might be coming from outside, Con Ed spokesman Bob McGee said. Con Ed dispatched two crews two minutes after the 9:15 a.m. call came in, McGee said. But they didn’t get there in time.

The tragedy brought the neighborhood to a standstill as police set up barricades to keep residents away. Thick, acrid smoke made people’s eyes water. Some people wore surgical masks, while others held their hands or scarves over their faces. Witnesses said the blast was so powerful it knocked groceries off store shelves.