By Ellen Wulfhorst and Sebastien Malo

NEW YORK (Reuters) - Workers may have "inappropriately" tapped into a gas line before a devastating explosion in New York City that destroyed buildings, injured 22 people and left at least two people unaccounted for, Mayor Bill de Blasio said on Friday.

A final determination of the cause of Thursday's blast could take a week as workers comb through debris to reach the basement where the explosion originated, the mayor said.

Two people who were in a sushi restaurant in the building where the explosion occurred are unaccounted for, he said. They were identified by police as Moises Lucon, age 27 or 28, and Nicholas Figueroa, 23. Figueroa is a college student who was eating lunch with a co-worker at the restaurant, his family told local media.

Police said a third person is unaccounted for but may not be connected to the blast, which destroyed three buildings and badly damaged a fourth.

At the scene on Friday, the rubble was smoldering and the buildings' remains were a tangled mess of bricks, wood, steel and broken glass. In all, 11 buildings have been evacuated, leaving residents of 144 apartments needing places to stay.

"You rarely see a scene of such devastation in the middle of a city like this," the mayor said.

Of the 22 injured, four people were in critical condition, the mayor said. Six firefighters were injured.

Investigators are looking into whether gas and plumbing work being done privately in one building triggered the explosion.

"It certainly looks like a possible option that something was tapped into inappropriately," he said.

An hour before the blast, Con Edison utility inspectors had been at the scene and determined that pre-existing work was not satisfactory, but the problems were not safety related, the mayor said.

A short while later, the owner of the sushi restaurant on the ground floor smelled gas and called the building's owner, who in turn called his general contractor, officials said.

The contractor and building owner's son went to the basement, where the blast occurred when they opened the door, officials said. Both suffered burns and were hospitalized.

The contractor, identified as Dilber Kukic, was one of 50 people arrested in February in a sweep of the city building and housing inspectors. He was accused of bribing an undercover investigator to dismiss violations at two properties, according to the Manhattan District Attorney's office. The hospitalized Kukic could not be reached for comment.

At his news conference, De Blasio voiced frustration that nobody called 911 or Con Ed to report the odor. Officials have repeatedly urged residents to report gas odors since an explosion in East Harlem killed eight people a year ago.

(Additional reporting by Laila Kearney; Editing by Susan Heavey, Will Dunham and Sandra Maler)