NEW YORK (AP) — As the smoldering ashes of the World Trade Center slowly yielded unimaginable carnage, investigators fanned out across the country Wednesday to track the conspirators who orchestrated an unprecedented day of terror from the air.

In one indication of the potential death toll, Mayor Rudolph Giuliani was asked about a report that the city has requested 6,000 body bags from federal officials. “Yes, I believe that’s correct,” said the mayor.

In another, 2,500 people visited a grief counseling center handling questions about missing family members Wednesday.

The last few floors that remained of the trade center’s south tower collapsed Wednesday afternoon in yet another cloud of thick smoke. No injuries were reported, but rescuers were evacuated from part of the area where the 1,350-foot titans stood.

Police and fire officials said there were problems with other “mini-collapses” among some badly damaged buildings nearby, and when the towers were destroyed, the Marriott World Trade Center hotel fell with them.

The search and rescue mission continued despite the problems.

The devastation turned the concrete canyons of lower Manhattan into a dust-covered ruin of girders and boulders of broken concrete. A Brooks Brothers clothing store became a morgue, where workers brought any body parts they could find.

The workers’ grim task was interrupted by brief epiphanies of life, when a fortunate victim was pulled alive from the wreckage of the steel-and-glass buildings. In all, five victims, three of them police officers, have been pulled from the wreckage alive.

In Washington, the Bush administration disclosed that the White House and Air Force One may have been among the targets of Tuesday’s devastation.

The investigation swept from a Boston hotel to Florida and points beyond — all in an attempt to determine who was behind the attacks in which two hijacked airliners barreled into the 110-story towers, a third dove into the Pentagon and a fourth crashed in western Pennsylvania.

President Bush condemned the onslaught as “acts of war” and NATO gave the United States its backing for a military response if the attacks were directed from abroad.

While investigators and diplomats moved forward in their tasks, progress for rescuers in New York was slow. Cranes and heavy machinery were used, but gingerly, for fear of dislodging wreckage and harming any survivors. Searchers with picks and axes worked slowly, too — sometimes when they opened pockets in the debris, fires flared.

Companies that leased space in the trade center began realizing the awful consequences of the violence. Thirty-eight people from Fred Alger Management Inc. were missing, including the company’s president, David Alger.

“The terrorist attack is a personal tragedy for my family as well as for all of our employees and their families,” said Fred Alger, the company founder and David’s brother.

Giuliani said the best estimate is that a “a few thousand” victims would be left in each building, potentially including 250 missing firefighters and police officers. Among the missing was John O’Neill, head of security for the trade center and a former FBI expert on terrorism.

There were 82 confirmed fatalities — a number that was sure to grow. Another 1,700 injuries were reported.

The four hijacked planes carried 266 people, none of whom survived. Officials from the military services said about 150 people, mostly Army personnel, were missing in the attack on the Pentagon. There had been estimates of 800 dead, but that was discounted by Defense Secretary Donald H. Rumsfeld.

Authorities had “specific credible information” that both Air Force One and the White House were targets, and that “the plane that hit the Pentagon may have been headed for the White House,” said Sean McCormack, spokesman for President Bush’s National Security Council.

There also was speculation that, in the case of the plane that crashed in Pennsylvania, the hijackers intended to jet elsewhere but were thwarted by passengers. One of them, Thomas Burnett, a 38-year-old business executive, told his wife by cell phone “a group of us are going to do something” before the crash.

The FBI investigation stretched across the country.

A Venice, Fla., man said FBI agents interviewed him and said that two men who stayed in his home last summer while training at a local flight school were among the hijackers.

Officials confirmed a car believed to belong to the hijackers was confiscated in Boston, where two of the hijacked planes took off, and that it contained an Arabic-language flight manual. Investigators also raided two Boston-area hotels believed to be used by the hijackers. Three suspects were detained but released.

Attorney General John Ashcroft said authorities had reviewed “numerous credible leads,” and were checking whether four separate cells of terrorists were involved. One set of hijackers is believed to have crossed from Canada and have ties to Osama bin Laden, the Saudi exile who authorities say is the suspected mastermind behind the attacks.

Bin Laden has been given asylum in Afghanistan, where international aid workers fled from the capital city of Kabul on Wednesday as residents worried about a possible U.S. military strike.

The nation’s Taliban rulers demanded to see evidence backing allegations that bin Laden runs a global terrorism network responsible for the hijackings.

In New York, the rubble at the trade center was taken by boat to a former Staten Island garbage dump, where the FBI and other investigators searched for evidence.

One volunteer, Peter Coppola, said he had found four dead bodies in his 24 hours of searching. “The air down there is totally toxic,” he said.

New Yorkers were told to avoid lower Manhattan and the financial markets were to remain closed at least until Friday.

Schools remained closed and the New York Yankees’ game was postponed, along with the rest of the major-league baseball schedule, including Thursday’s games. Many other sporting events were either canceled or postponed.

Joe Allbaugh, head of the Federal Emergency Management Agency, toured the site of the tragedy with local and state politicians as the federal government begins assessing what more can be done to help with search, rescue and restoration.

Wednesday night the Empire State Building and Pennsylvania Station were partly evacuated because of fears bombs had been planted. Police and Amtrak officials said both incidents were unfounded.

Federal officials partially lifted a ban on nationwide air travel, allowing flights that had been diverted on Tuesday to finish their journeys and empty planes to be moved around. All other flights remained grounded.

People still clung desperately to the hope that their missing friends and family members were somehow alive. At St. Vincent’s Hospital, where hundreds of victims were treated, a sobbing Annelise Peterson walked in a daze, clutching pictures of her boyfriend and brother.

Peterson asked if anyone had seen either. No one could tell her yes.

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