NEW YORK - Searching with hands and dogs through scoops of rubble from three apartment buildings leveled in an apparent gas explosion, emergency workers painstakingly looked for signs of two missing people Saturday, though authorities acknowledged the chances were slim.

Meanwhile, investigators worked to piece together exactly what caused the blast that injured 22 people in Manhattan's East Village.

Officials estimated it could take a week of 24-hour-a-day work to sift through the heap of loose brick, wood and debris.

"It's going to be slow and arduous," Fire Commissioner Daniel Nigro said Saturday. Emergency workers were using heavy machinery to dig out rubble and place it in the street, where each scoop is examined manually and sniffed by dogs, he said.

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 showed photos of their loved ones and asked for help.

"We have just been walking down the streets, one by one," brother Zacarias Lucon told the Daily News of New York on Friday. "We are just so exhausted and upset."

Figueroa's relatives said they were holding out hope.

"My brother is strong," 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."

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

As of Saturday, no one else was believed to be missing related to the explosion, which sparked a raging blaze that took hundreds of firefighters to quell. Mayor Bill de Blasio visited a firehouse Saturday to thank some of them.

As some of the several evacuated buildings near the explosion site 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, de Blasio said Friday.

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.

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.