NEW YORK - Mayor Michael Bloomberg announced a shakeup among Lower Manhattan fire officials on Monday and hinted at more fallout over a deadly fire in an abandoned ground zero building, acknowledging the city failed on numerous levels to protect firefighters from the blaze.

Officials also said Monday that the cause of the fire appeared to be from cigarette smoking, likely by workers who were demolishing the former Deutsche Bank tower.

The building had become a toxic quagmire due to damage sustained six years ago during the World Trade Center attack.

Bloomberg was unusually frank about the failures in the building, and signaled that no agency that had a hand in the project would be spared from scrutiny.

He said the fire department failed to conduct the required regular inspections at the tower as workers dismantled the former 41-story building floor by floor.

Had they performed those checks, they might have seen numerous conditions that contributed to the Aug. 18 fire, including a broken water supply system, a maze of sealed-off stairwells, combustible debris throughout the building and signs that workers regularly ignored the no-smoking rule on site.

Three fire officials were relieved of their commands and sent to work at headquarters - reassignments that Bloomberg warned could be just the beginning of the city's administrative response, including possible disciplinary actions.

Bloomberg said it was “not excusable” that the department failed to properly inspect the building, especially after repeated urging from at least one fire official who spotted numerous potential hazards in the skyscraper and sent memos about his concerns.

The mayor said the city is obliged to reduce risks to firefighters wherever possible, adding, “As a city I think it's safe to say that we failed to do that.”

“I'm not interested in fingerpointing,” he said. “I simply want to fix what is broken and that's why we've spent a lot of time over the last 10 days trying to account for those failures.”

He noted that the Manhattan district attorney and state attorney general are investigating.

The city also asked the FBI for help in figuring out how and when the building's water supply network, known as the standpipe, was broken.

Fire marshals investigating the blaze found pieces of the standpipe unattached in the building's basement, and the valve was also discovered to have been turned off at some point.

Portions of the pipe were sent to be analyzed by FBI metallurgists in Quantico, Va.

After the fire broke out on the 17th floor, more than 100 firefighters rushed into the tower to battle the blaze, including firefighters Robert Beddia and Joseph Graffagnino, who died of cardiac arrest from smoke inhalation.

Bloomberg said the city believed the response was appropriate, considering what officials knew at the time.

“But it's what they didn't know that contributed to the enormous difficulties they encountered,” he added.

Among those unknowns: Some of the building's floors were still being decontaminated of asbestos and other toxins, and were sealed off to prevent leakage.

Officials believe that this created atypical air pressure that caused the flames to behave differently.

High-rise fires generally burn upward, but officials said this fire did the opposite - it was quickly sucked downward from where it began on the 17th floor, putting the base of operations on the 14th floor at unexpected risk.

It was a situation that might have been noted in a pre-fire plan for the building, which apparently was not created, despite urging from Battalion Chief William Siegel in 2005. Fire Commissioner Nicholas Scoppetta said fire officials instead relied on a common citywide plan for fighting high-rise fires.

Bloomberg on Monday said he had ordered the city's Department of Environmental Protection to devise a procedure for notifying the Fire Department when decontaminations are taking place that could affect fire response.

He said the city was also investigating whether the Department of Buildings bore responsibility in the inspections failures.

Bloomberg's unusually harsh stance on Monday marked a departure from previous disasters, like blackouts, subway breakdowns and other tragedies, in which he has been known to vigorously defend those who face the harshest criticism.

The fire officials who were reassigned include a deputy chief, who received memos from Siegel, as well as a battalion chief for the Lower Manhattan area where the building stands.

A captain at the local firehouse, which is next door to the toxic tower, was also moved.