Removing the remaining debris from the massive grave that was once the World Trade Center could take up to a year, Mayor Rudolph Giuliani said Friday.

Although the Twin Towers collapsed in mere minutes, the cleanup will be lengthy due to the difficulty of removing structures "that have been driven into the ground."

As debris is cleared from the 16-acre site, human remains continue to be removed for identification.

Giuliani said the official count of people missing in the still-smoldering rubble has dropped to 5,960 from nearly 6,400. The number decreased after victim lists, compiled primarily from missing persons reports and information from foreign consulates following the Sept. 11 terrorist attack, were rechecked. There are 305 confirmed deaths.

"If you stare at the debris and think of all the people buried there it will get to you," said Peter Russo, 55, a carpenter from Old Bridge, N.J., who was building sheds for crews working at the site. "I have to look at it as a job, that the cleanup and rebuilding needs to be done."

Tons of steel and concrete cover the blocks where the 110-story towers once stood. Despite the delicate search for victims, crews have begun assembling giant cranes capable of lifting hundreds of tons.

"Every day we come down here, another 50 feet off the pile is gone," said Brian Bowman, 26, a Verizon worker restoring phone service near the site. "Every day we come down here, there's a new crane."

On the 10th-floor roof of a nearby building, a color guard ceremony was held Thursday to honor the veterans who died in the collapse.

Broken glass littered the rooftop where the color guard of active Marines in uniform and former or reserve Marines from the Police Department, all in hard hats, flew the flags of the United States and its Marine Corps, Navy, Air Force, Army and Coast Guard.

As the ceremony ended, the color guard and some workers below snapped salutes. On the building, a banner read: "We Will Never Forget."

"We don't know how many veterans were killed in this terrorist act," Marine Maj. David Andersen said. "We may never know. But we do know there were many among the rescuers and the other victims and this is to honor them all."

Away from the site, mandatory carpooling rules intended to ease traffic jams that have slowed the city since the attacks were in place for a second morning Friday. Single-occupant cars were banned from coming into Manhattan via several East River bridges and tunnels

The rules for the first time included the busy Lincoln Tunnel linking Manhattan to New Jersey, but authorities said the hour-long backups that stretched two miles to the New Jersey Turnpike were not unusual.

The traffic jam in Weehawken to police Sgt. Mitchell Chasmar.

"Inbound is stopped, and westbound is crawling. I advise walking," Chasmar said. "It doesn't look any worse to me than a normal rush-hour."

Tunnel officials said volume was down, from 5,500 New York-bound vehicles per hour between 7 a.m. and 9 a.m. on a typical Friday before the attacks to 4,000 Friday. General manager Kos Skruodys said he didn't know whether it was because of the ban or because it was the day after the Jewish holiday of Yom Kippur.

Giuliani said the 6 a.m. to noon ban would be evaluated after Friday and there was a possibility they might be extended.

On Saturday, the Empire State Building's observation deck was to reopen. Except for a brief trial period Sept. 15, the 86th floor observatory had been closed to the public since the terrorist attacks.

The Associated Press contributed to this report.