The water had been turned off that day to allow for maintenance work in the 16-story building, which was built in 1964 and contains 275 apartments.
Theresa Racht, a lawyer for the co-op board, told AFP that it appeared to have been a freak accident.
"This is a horrific incident. Everybody feels terrible that such a thing could have happened," she said.
"It certainly makes me think twice about flushing the toilet when the water's been turned off."
She said four toilets in the building exploded but that no particular evidence of wrongdoing had yet been found.
"It has never happened before and certainly nothing has happened since," she said.
"The only conclusion anybody has reached -- and they're still investigating -- is that there was a buildup of air pressure in the pipes so that when it came back on, the pressure was just pushed through the pipe and caused the explosion."
"This could be very well what we call a true accident."
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