Manhattan District Attorney Robert M. Morgenthau announced Tuesday that his office was investigating a fire in a vacant office tower at Ground Zero where two firefighters lost their lives last week.

Prosecutors have been in touch with fire marshals from the New York City Fire Department and will work with the NYPD and other agencies to determine if any criminal violations occurred in connection with the deadly Aug. 18 blaze at the Deutsche Bank building, Morgenthau's office said.

Earlier Tuesday, Mayor Michael Bloomberg defended the Fire Department's decision to send dozens of firefighters into the empty building, which was in the process of a complicated demolition.

"You just can't let a fire go out of control," he said. "The alternative of just standing aside and letting a fire go is not something that was ever considered nor should it have been considered."

A key part of the investigation into the Fire Department's response will focus on the fact that the building's water supply system was not operational. Investigators found a chunk of the standpipe unattached and lying on the floor in the basement, the city said.

Firefighters tried to get the water going after arriving at the scene, but because of the broken pipe, the network flooded the basement and they had to use other means to get water to the flames on the upper floors.

• PHOTO ESSAY: Fire Erupts at Skyscraper at Ground Zero

"Nobody at this point knows if that [the standpipe] was a contributing factor to the two tragic deaths or not," Bloomberg said. "That's what an investigation will be for."

The troubled building had been plagued with citations before the fire, and received another violation for failure to maintain the standpipe system after investigators made the discovery.

The former Deutsche Bank office building has been a toxic mess since it was damaged on the morning of the World Trade Center attack six years ago, and was being dismantled floor by floor. It once stood 41 stories, but demolition crews had whittled it to 26 by Saturday, when the fire erupted.

The city also said Monday that fire investigators had determined the blaze began in an area on the 17th floor where workers would stop before entering and exiting a chamber for decontamination. Fire marshals spoke to eyewitnesses who said workers would smoke and put out cigarettes in the area, which was near the construction elevator that they used to access the floor.

The city said there was also some electrical equipment there, including hot water heaters for decontamination showers.

Bloomberg said in that investigators still did not know how the fire started.

"At this point we do not know the cause of the fire, but full and comprehensive investigations are under way," he said. "We are using every possible resource to find out how this fire started and what went wrong."

The mayor said separate investigations were ongoing to determine how the fire started and what circumstances led to the deaths of the two firefighters. Officials also were trying to sort out the confusion about who was responsible for the standpipe and other issues — private contractors are working on the state-owned building, while multiple local, state and federal agencies have a hand in the decontamination and deconstruction. The effort was described by the city as "unusually complex."

In working buildings, the fire department is responsible for checking the water flow in standpipes every five years, according to fire department spokesman Jim Long. Building owners typically maintain them in between.

The city could not say on Monday when the water network had last been tested, but the fire department said the building had been issued at least one other violation related to standpipe problems.

The skyscraper's owner, the Lower Manhattan Development Corp., bought the tower three years ago and took over the removal project. As the demolition proceeded, the city Department of Buildings issued a separate permit for each floor before it could be taken down.

With each floor permit, the Department of Buildings visually inspected the valves and caps for the standpipe in that area, but those inspections did not include testing the water flow. The last permit was as recent as July 31, for the dismantling of the 26th floor.

The Department of Buildings said it was still sorting out its inspection records but noted that its agency was only involved in the floors that were being dismantled. The fire started much lower and was limited to floors 13 through 18.

The LMDC said it was still collecting information about what may have gone wrong.

"Two firefighters lost their lives — we're all trying to figure out what happened at this time," LMDC chairman Avi Schick said. "Nobody knows."

Apart from the water problems, the LMDC had racked up a number of citations before the fire from city building inspectors, for complaints including debris falling from the building and excessive amounts of combustible debris and plywood stacked around the site.

Just weeks ago, buildings inspectors found that torch work being performed on the 28th floor was causing sparks to rain down near combustible material, though that was ruled out as a cause of the weekend fire since crews apparently had not been working with torches.

Many of the issues raised in the complaints and violations foreshadowed complications that firefighters faced when they responded to the blaze. Along with the water problems, emergency responders had difficulties navigating around the debris inside the building, and in many places it was easy to get disoriented, they said.

Firefighters Joseph Graffagnino and Robert Beddia got lost on the 14th floor as their air tanks ran out. They inhaled smoke and died of cardiac arrest; their funerals are Thursday and Friday. More than 50 firefighters suffered minor injuries.

More than 10 floors of the skyscraper were sealed off with polyurethane to keep toxic dust containing asbestos, lead and trade center materials from leaking out into the air. Gov. Eliot Spitzer theorized that the protective materials "may in fact have made this fire harder to fight."

He said the polyurethane was there because of federal Environmental Protection Agency requirements; a spokeswoman for the agency said it was a state labor rule.

Officials also said firefighters were hindered by plywood barriers that blocked stairway access to the floors that had yet to be decontaminated. Some air-quality tests were still pending on Monday, but results for many samples had come back normal.