NEW YORK - Someone may have improperly tapped a gas line before an explosion that leveled three apartment buildings and injured nearly two dozen people, Mayor Bill de Blasio said Friday as firefighters soaked the still-smoldering buildings and police searched for at least two missing people.

"There is a possibility here that the gas line was inappropriately accessed internally" by people in one of the destroyed buildings, but officials need to get access to its basement to explore it further, de Blasio said.

The number of people injured in Thursday's blast rose from 19 to 22, with four critically injured. Police were searching for at least two people: Nicholas Figueroa, a bowling alley worker who had been on a date at a sushi restaurant in the building where the destruction was centered, and Moises Lucon, a worker there. Authorities also were exploring whether a third person was unaccounted for, Chief of Detectives Robert Boyce said.

Preliminary evidence suggests an explosion amid plumbing and gas work inside the building was to blame.

Inspectors with utility Consolidated Edison had been to the East Village building to check on ongoing work to upgrade gas service. The utility said the work didn't pass inspection, so gas wasn't introduced to the line, and inspectors gave instructions and left. Con Ed said inspectors didn't smell gas.

But 15 minutes later, the sushi restaurant owner smelled gas and called the landlord, who called a general contractor, Boyce said. No one called 911 or Con Ed, however, de Blasio said.

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.

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 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 investigation publicly and spoke on the condition of anonymity.

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