A blast from a suspected gas leak set off a seven-alarm inferno that burned four Manhattan buildings, collapsed one, partially collapsed two others, and injured 19 people, four of them critically, New York City officials said.

The explosion, which injured four firefighters, rocked the block where employees of a private contractor were adding a new plumbing and gas system to an existing one inside a sushi restaurant at 121 Second Ave., authorities said.

Even hours after the blast on Thursday night, firefighters were hosing down the smoldering, smoking scene, where debris was still falling, as heavy rain poured down at times. They also used water pressure to take down leftover facades as displaced residents searched for shelter and Con Edison workers investigated.

"Preliminary evidence suggests a gas explosion," Mayor Bill de Blasio said earlier at the scene in the East Village.

But he warned, "until we know what happened here, we cannot pass judgment."

In fact, on Thursday night, Con Edison released a statement saying its personnel had been at the building to "evaluate work the building plumber was doing inside 121 2nd Ave. in connection with a gas service upgrade. The work failed our inspection for several reasons, including insufficient spacing for the installation of the meter in the basement.

"We had no reports of gas odors in the area prior to the fire and explosion," the statement continued. "A survey conducted yesterday of the gas mains on the block found no leaks. We continue to work with all agencies on the investigation into the cause, and we are praying for the recovery of all the injured."

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The boom was heard about 3:15 p.m., following the 2 p.m. Con Edison inspection that gave failing marks to the new work, preventing any gas from flowing through the new lines, authorities said.

"It sounded like two tractor-trailers hitting head on," said Jason Birchard, whose family owns Veselka, a restaurant a block north.

He and his employees raced out to a smoky scene, and saw debris everywhere, then flames 10 to 15 minutes later.

"My first visual was multiple people down in the street . . . on Second Avenue on the sidewalk," he said. "They were cut by flying glass, it looked like, some people just lying on the ground from the impact of the explosions, bleeding."

Ataur Rahman, 57, general manager of the Dallas BBQ at one end of the block, said his building shook from the explosion. He ran outside to see what had happened.

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"The storefront was completely empty," Rahman said. "The glass was all over the street."

There was just debris where the Japanese restaurant had been, he said: "I saw the whole storefront was on the street."

Two bloodied victims were lying in the street, he said, and neighbors began stopping traffic rolling down Second Avenue.

Tobarka Hassan, a waiter working at a nearby Indian restaurant, said he rushed out at the sound of a huge explosion and spotted a woman on the third floor of one building fleeing to safety using the fire escape of an adjacent building.

"Everything was falling down," Hassan said. "Fire was coming from the basement to the top of the building. I felt the heat from here, at least half a block away."

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The first of 250 firefighters arrived three minutes after the first 911 call came at 3:17 p.m., said FDNY Commissioner Dan Nigro: "They certainly didn't expect to see the explosion blow the front of 121 across the street."

Flames consumed the upper floors of the five-story buildings at No. 121 and 123.

Then just before 4 p.m., as firefighters had feared, No. 123 crumbled. A huge gray plume of smoke and dust mushroomed up, but no firefighters were hurt. No. 121 also partially collapsed.

The inferno spread to adjacent buildings, No. 119 and 125 and later, the FDNY said, 119 also partially collapsed.

De Blasio told reporters that subduing the fire was a tough battle.

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"This is a complex and difficult operation they're mounting here," he said. Firefighters are "obviously doing everything they can to search for anyone who still may be in those buildings, but also to ensure that there's no spread of fire to the surrounding buildings."

Of the four critically injured, two had burns to their airways and one was knocked unconscious, Nigro said.

City officials said they did not have any reports so far of people smelling gas just before the blast.

Con Edison crews shut off gas to the area, and Gov. Andrew M. Cuomo said the state Department of Public Service was on the scene to monitor the utility's performance.

He also said the state agency will conduct a "full investigation" to determine the cause of the explosion.

"We will continue to monitor this tragedy and do whatever is needed to support the ongoing response and recovery in the days ahead," he said in a statement.

Joe Esposito, the city's emergency management head, said authorities were concerned about air quality and warned people to keep windows closed and limit time outside.

He said a "debris task force" would try to clear debris out as soon as possible.

Red Cross officials set up a shelter at a nearby school, where Anna Ramotowska, 26, and her roommate, Lucie Bauermeister, showed up.

Ramotowska said after lunch she had returned to their third-floor apartment at 129 Second Ave. when they felt the blast.

"It felt like an earthquake," Ramotowska said.

They grabbed their dog, their phones and wallets, and ran out, where they saw glass all around the street and people, young and old, trying to escape down a mangled fire escape.

The roommates said they will stay with friends.

"We want to know what is going on," Ramotowska said, "and when we can go back."

Some 79 adults and one child had registered for services at a Red Cross disaster center at PS 63, said Josh Lockwood, regional CEO for the American Red Cross Greater New York region.

"People are stunned. Most people are in a state of shock. They're processing the days events. They're grieving over the loss of a home. They might have a pet that's missing," Lockwood said.

With Anthony M. DeStefano, Darran Simon, Alison Fox, Matthew Chayes, Maria Alvarez, Ivan Pereira, Robyn Spector and Dan Rivoli