Amid the grim accounting of the dead and injured from the airborne onslaught that toppled the World Trade Center and blasted the Pentagon, authorities said Wednesday they believed the terrorists had had other targets ? the White House and Air Force One.

?We had specific credible information that both were intended terrorist 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.

The revelation came on a day when investigators pursued leads to Canada, Massachusetts, Florida and beyond, spurred by the horrors that continued to unfold in the ashes of New York and suburban Washington.

In Florida, FBI agents questioned a man about two men who stayed with him while getting flight training last year. Charlie Voss, a former employee at Huffman Aviation in Venice, said FBI agents who interviewed him at his home told him that authorities found a car at Boston?s Logan Airport registered to the two men.

?They informed us individuals who had crossed our path were involved yesterday with the airplane in the tragedy at the World Trade Center,? Voss said.

Voss said the agents identified the men as Mohamed Atta and someone known as Marwan.

The FBI in Miami issued a national bulletin for law enforcement agencies to look out for two cars. Records with the Florida Division of Motor Vehicles show that one of the vehicles the FBI was pursuing ? a 1989 red Pontiac ? was registered to Atta.

Federal officials, speaking on condition of anonymity, also said they were investigating whether one group of hijackers crossed the Canadian border at a checkpoint and eventually went to Boston?s airport, where the two airliners that brought down the Trade Center took off.

The financial capital remained closed after the attacks. Federal officials partially lifted a ban on 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.

On Wednesday morning, Mayor Rudolph Giuliani said there were 41 known deaths so far ? clearly, a tiny fraction of the dead ? and 1,700 known injuries. He said 259 uniformed officers, including police and firefighters, remained unaccounted for.

The mayor said rescuers were in contact with one person buried in the rubble. Three police officers and a woman had been taken from the wreckage, alive, Giuliani said.

Authorities gave reporters their first close-up look at the site, and this is what they saw: Only about seven stories of the north tower remained, its girders bent outward. The south tower was a two-story-high heap of rubble.

Progress was slow. Cranes and heavy machinery were used, but only 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.

President Bush declared the attacks ?acts of war.? He said he would ask Congress for money for recovery and to protect the nation.

The announcement that the White House was believed to be a target explains why Bush did not immediately return to Washington from Florida on Tuesday to take charge of crisis operations run from the White House basement. Instead, he went to Louisiana and Nebraska before returning home.

The focus of the investigation was on Islamic terrorist Osama bin Laden, who denied involvement, though he ?thanked Almighty Allah and bowed before him when he heard this news? of the attacks, according to a Palestinian journalist.

But sources told The Associated Press that investigators were looking into the possibility that four different terrorist cells were involved.

?This could have been the result of several terrorist kingpins working together. We?re investigating that possibility,? one law enforcement official said.

FBI agents searched a room at the Westin Hotel in Boston?s Back Bay; they said the room was vacant, but they found information linking it to a name on the manifest of one of the hijacked flights. They would not identify the man.

Law enforcement officials were said to be looking at possible bin Laden supporters in Florida. They were aided by an intercept of communications between his Florida supporters and harrowing cell phone calls from victims aboard the jetliners before they crashed.The government went back to work Wednesday, its political leaders, diplomats and soldiers leaving no doubt the terrorist assault will be answered. ?We will go after them,? Secretary of State Colin Powell vowed.