Hope dimmed as emergency workers continued their search Saturday for two people still missing after an apparent gas explosion leveled three Manhattan apartment buildings and investigators worked to piece together what exactly caused the blast that injured 22.

Dogs sniffed for anyone possibly still trapped beneath the heap of loose brick and rubble two days after the explosion. Detectives issued posters seeking information on the whereabouts of two men believed to have been in the sushi restaurant on the ground floor of one of the collapsed buildings: 26-year-old Moises Lucon, who worked at the restaurant, and 23-year-old Nicholas Figueroa, a bowling alley worker who had been there on a date.

Their families frantically searched, showing photos of their loved ones and asking for help.

"We have just been walking down the streets, one by one," brother Zacarias Lucon told the Daily News of New York. "We are just so exhausted and upset. I don't know what happened to him."

Lucon moved to New York about six years ago from Guatemala.

Figueroa's relatives said they were holding out hope.

"My brother is strong," said brother Neal Figueroa told reporters. "Even if he is still in the rubble, I know he would still be in a predicament to get himself out, and so I'm just praying for that."

As of Saturday, no one else was believed to be missing related to the explosion.

But hope was dimming. When asked about whether anyone would have survived, Joseph Esposito, head of the Office of Emergency Management, said: "I would doubt that very seriously."

While Mayor Bill de Blasio visited a firehouse to thank some of the hundreds of firefighters who battled Thursday's massive blaze, heavy equipment plied the mound of rubble Saturday. As some of the several evacuated buildings nearby were declared safe for residents to return, Micha Gerland stood at a police barricade and surveyed the remains of his apartment.

"I still don't believe it," said Gerland, 37, a restaurant manager who escaped with nothing but his wallet, his phone, his keys and the clothes he was wearing. "Who thinks that something like that happens?"

It's possible that someone improperly tapped a gas line amid ongoing plumbing and gas work in one of the destroyed buildings, though investigators need to get into the basement to learn more, the mayor said.

Inspectors from the utility company Consolidated Edison had visited that building Thursday about an hour before the explosion and determined work to upgrade gas service didn't pass inspection, locking the line to ensure it wouldn't be used and then leaving, officials said.

Fifteen minutes later, the sushi restaurant's owner smelled gas and called the landlord, who called the general contractor, Boyce said. Nobody called 911 or Con Ed.

The contractor, Dilber Kukic, and the owner's son went into the basement and opened a door, and then the explosion happened, burning their faces, Boyce said. Kukic, who has pleaded not guilty to an unrelated charge of bribing a housing inspector, declined through his lawyer to comment on the circumstances surrounding the explosion.

The building had an existing gas line intended to serve the sushi restaurant; the work underway was to put in a bigger line to serve the entire building, Con Ed President Craig Ivey said. As for whether the largely vacant apartments were getting gas from the existing line, "That's a great question," he said.

De Blasio wouldn't say more about why officials believe the existing gas line might have been tapped. But the building had a history: Con Ed found an unauthorized gas pipe there in August after getting a report of a gas smell, according to a city official briefed on the information. The official wasn't authorized to discuss the ongoing investigation publicly and spoke to The Associated Press on the condition of anonymity.

The pipe was gone when Con Ed checked again 10 days later, the official said.

The landlord didn't immediately respond to calls and emails Friday and Saturday.

Dozens of neighbors have been displaced because of the blast, and a fourth building badly burned after the explosion remains at risk of collapsing, officials said.

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Associated Press writer Kiley Armstrong contributed to this report.