MORE ON THIS STORY FULL COVERAGE • America Attacked VIDEO • Hijacked Plane Hits First Tower • Hijacked Plane Hits Second Tower • Complete Video Coverage of the Attacks IN DETAIL • Interactive: Terror Strikes America COMMUNITY • What Americans Are Saying

At a news conference this morning, New York Mayor Rudolph Giuliani dismissed reports that a "knocking" sound from a potential survivor had been heard in the midst of the debris.

Nonetheless, Giuliani said, "We're going to continue to search for people and look for people, at the same time realizing the losses here are staggering."

Not Giving Up on Survivors

No survivors have been found since Wednesday, a day after hijacked planes smashed into the World Trade Center and the Pentagon, and another hijacked aircraft crashed in rural Pennsylvania. But officials are still calling their efforts a "search and rescue" operation.

On ABCNEWS' This Week, Giuliani said he would wait "as long as it takes" before declaring that no more survivors are likely to be found, based on input from rescue experts and the Federal Emergency Management Agency.

Giuliani did not specify a specific time when he thought that could happen, and said rescuers would first explore all possible air pockets in the wreckage.

According to New York police, 5,097 people are still missing. The official death toll is at 180, with 115 of those people now identified. Twenty-four firefighters were among those dead.

FBI Combs Area

The FBI has been conducting a block-by-block "grid search" through devastated lower Manhattan, looking for the voice and flight data recorders of the two aircraft flown into the twin towers.

Investigators discovered the passport of Satam al Suqami, one of the terrorists aboard American Airlines Flight 11, the first plane to hit the World Trade Center.

Rescue workers have now cleared away 21,000 tons of debris, using 1,200 truckloads. But when they tunnel into the wreckage, it lets in more air and often has the effect of feeding oxygen to the smoldering fires and hot debris.

The cleanup operation has also cleared the surrounding streets of debris in an effort to make the hauling process go faster.

DNA Samples Sought

Families of people missing from Tuesday's attacks have been gathering personal items to help investigators identify victims in New York and Washington. Giulliani has asked families and friends of the missing to hand over personal materials that might help the medical examiner recover DNA samples to identify their bodies.

Many people looking for a missing friend or relative lined up at the 69th Regiment Armory in Manhattan with items like hair combs and tooth brushes that could aid authorities in determining whether their loved ones were trapped in the destruction of the twin towers.

Hoping to find more of the trapped people, FEMA has made arrangements to collect cell phone numbers of missing New Yorkers. Every cell phone company in the nation has agreed to "pulse" those numbers in hopes that either someone will answer, or that they can be located in the rubble.

While the heroic recovery efforts moved ahead, activity of a decidedly non-heroic nature started rearing its head — looting.

New York Police Commissioner Bernard Kerik said one person was arrested for attempted burglary at Brooks Brothers, several have been arrested for trespassing, and one person was arrested for trying to steal a fireman's jacket.

Giuliani has also warned people to be wary of charity scams and people who solicit donations through telemarketing operations. The mayor said no legitimate organizations were doing so.

Grim Task at the Pentagon

In Northern Virginia, across the Potomac River from Washington, crews continued the grim job of sifting through the debris at the Pentagon. Salvage crews expected to remove more bodies throughout the day from the jetliner that drove a hole through the Pentagon's walls.

The government said that 188 people are unaccounted for since the attack on the Pentagon, while 83 sets of remains have been taken from the site and 77 sets have been sent to Dover Air Force Base in Delaware for identification.

Amid the daunting task, some good news emerged. The section of the building hit by the plane was recently bomb-proofed, and engineers said the renovations may have saved several hundred lives.

Even after the plane hit the building, the newly renovated Pentagon walls did not immediately fall, giving workers 35 minutes to flee before the floors collapsed. Engineers say if terrorists had hit another corner of the building, hundreds more may have been killed.