"This could have been an even worse disaster than it already is," Fire Chief Nicholas Scoppetta said.

Power company Consolidated Edison said an employee had been in the basement of an adjacent building responding to a complaint about a smell of gas at the time of the blast. The employee was unhurt.

The utility had been at the Bartha building June 8 after a routine check found a gas leak on a pipe in the basement. The gas was shut off, and Nicholas Bartha was asked to get the pipe fixed, spokesman Joe Petta said. The gas was turned back on after the utility ensured the leak was fixed.

Yaakov Kermaier, 36, a resident in a building next door, said he was outside when he heard "a deafening boom. I saw the whole building explode in front of me.

"Everybody started running, nobody knew what was coming next," he said. His nanny and newborn escaped from their next-door apartment unharmed.

The building housed two doctors' offices. Authorities said a nurse who was supposed to open one of the offices arrived late, narrowly missing the explosion.

Bartha was apparently the only person who lived in the building, Scoppetta said.

Thad Milonas, 57, was operating a coffee cart across from the building when he said the ground shook and the building came down. He said he helped two bleeding women from the scene.

"In a few seconds, finished," he said. "The whole building collapsed."

TV host Larry King, who had been in a hotel room nearby, described the explosion to CNN as sounding like a bomb and feeling like an earthquake.

"I've never heard a sound like that," King said.

The building is in an upscale neighborhood where the 2000 Census put the median home price at US$1 million. The area was once synonymous with high-society types like J.P. Morgan and William and Cornelius Vanderbilt.