Americans cautiously tried to get back to the routines of their previous lives on Thursday, two days after the United States was jolted into a darker world.

While the national air system tried to return to regularity in fits and starts and the White House opened to tours, little signs — like the National Football League canceling its Sunday games — proved that life in American was anything but normal.

In the most profound attempt to retrieve some of the way of life Americans enjoyed only last week, Transportation Secretary Norman Y. Mineta announced that the nation's airspace was reopened to commercial traffic at 11 a.m. Thursday on a case-by-case basis.

"In the weeks and months ahead we’ll do all we can to ensure safety is place," Mineta said. "We will not let the enemy win the war by restricting the mobility of America’s people. We’re going to restore normalcy as much as we can."

But air travel won't be the same: Curbside luggage check-in is forbidden, boarding areas are restricted to original passengers only and authorities are eyeing all vehicles. Airlines are allowed to move empty aircraft. Personal aircraft will still be grounded.

And foreign airlines chafed when all but U.S. carriers were barred from the U.S., Reuters reported. Dutch KLM had to cancel two flights and  TAP-Air Portugal and Italian Air Italia airliners bound for New York had to return home. Foreign airliners were allowed to depart the U.S., however.

And just hours after the skies were opened up again, New York's three major airports once again forbad landings because of an FBI investigation in which three men were held "for questioning."

Peter Yerkes, a spokesman for the Port Authority, which runs the airports, said that planes in the air were being allowed to land, but no planes were allowed to take off. The hold went into effect shortly before 5 p.m. He declined to give further details.

Mineta said even more serious measures, such as putting armed marshals on flights, were being discussed.

Regardless of whether that step is taken, U.S. marshals, the U.S. Customs Service and the Border Patrol will be part of increased security on the ground at airports, Justice Department spokeswoman Mindy Tucker said.

The Air Transport Association even said the FAA should consider taking over the passenger screening process rather than leaving it to the airlines.

When individual airports open depends on how quickly they meet the new security requirements.

Hartsfield Atlanta International Airport resumed flights. Lambert Airport in St. Louis was to open at noon and Kansas City International Airport was back in business at 10 a.m. Los Angeles International expected to be on track sometime in the afternoon.

But others were not ready to return to flying. In Washington, D.C., Ronald Reagan National Airport wasn't planning any flights coming or going, and Dulles limited flights to those that were taking diverted planes to their original destinations. And at one of the nation's busiest airports, Chicago's O'Hare International, United Airlines officials said they didn't expect to be ready until at least 6 p.m.

At Boston's Logan airport, where the two planes that hit New York originated, officials said they will not reopen until the airport has met the FAA's security requirements.

At Montreal's Dorval Airport, all flights to the United States were still listed as cancelled as the 11 a.m. EDT resumption time passed.

"We're being told that there are no flights on Northwest. Right now, they can't guarantee anything for a week," said Ana Belda of Alicante, Spain, who was waiting with her husband, two children and two friends for a flight to Detroit.

At least some regularly scheduled United Airlines flights were expected to begin at 7 p.m. EDT Thursday, and some scheduled flights on American Airlines and TWA after 4 p.m. EDT.

"We expect the return of our full schedule of service to take several days," American said.

Delta Airlines said "very limited operations" would start sometime after noon EDT Thursday. Continental had canceled all regularly scheduled flights for the day, but planned to offer special service in cities such as San Francisco and Cleveland, "where we see demand," spokeswoman Erica Roy said.

The complete ban, the first of its kind in American history, had forced more than 120 flights to Canada and the displacement of thousands of passengers.

But on the FAA Web site, Mineta promised America will still be a land of free travel:

"Each American must know that we will restore our national transportation system to a safe and efficient status as quickly as possible. … In a democracy there is always a balance between freedom and security. Our transportation systems, reflecting the values of our society, have always operated in an open and accessible manner. And, they will again."

'Not Business As Usual'

In the badly bitten Big Apple, bridges and tunnels that closed, cutting off Manhattan from the rest of the world, were reopened. Nationwide, except in lower Manhattan, schools reopened. And in a symbolic move, tours of the White House resumed Wednesday, and the 400 visitors each received small American flags as they left.

But America’s financial centers were still in utter disarray, with the New York Stock Exchange closed down until Monday. There was also no reliable report of how many members of New York's vast financial community had been lost in the attacks.

The New York Board of Trade, which was significantly damaged in the attack, was closed, and its Long Island backup facility will not open until Sept. 17. Some of the world’s major financial firms were in the building, including Morgan Stanley.

In New York City, both landline and cellular phone service was patchy at best, as phone companies struggled to deal with an influx of worried calls into the area even as the telecommunications hub at and near the World Trade Center was critically damaged.

In Washington, D.C., all federal agencies were open, but personnel were permitted to take unscheduled leave. Bridges into the city were open, but travelers were told to expect delays.

In spots that weren’t hit Tuesday, both government and commercial offices shut down abruptly. Atlanta’s Coca Cola headquarters closed up shop, as did Walt Disney theme parks, San Francisco’s pyramidal TransAmerica Tower and Minnesota’s Mall of America. All of the 3,700 or so Starbucks in the country shut down.

The devastation even affected America's national pastime. Major League Baseball canceled all of its games Tuesday, and on Wednesday announced no games would be played on either Wednesday or Thursday. It was the first time the organization had postponed a full schedule of regular-season games since D-Day in 1944.

Government offices around the country were closed down after the attacks Tuesday.

In California, Gov. Gray Davis had shifted his operations from Los Angeles to a new site outside Sacramento, but all government offices besides federal offices were to reopen Wednesday. Madonna cancelled her concert and the Latin Grammys were postponed.

In Chicago, the nation’s tallest building, the Sears Tower, which had been evacuated after the attacks, reopened Wednesday, as did most government buildings. But parking was forbidden on major roads near the Loop and near city, state and federal buildings. The City That Works’ four exchanges agreed jointly to remain closed a second day.

In Florida, Gov. Jeb Bush vowed to get the state government back to work.

"The best way to defeat terrorism is to get back to work serving our people and our state," he said. "Barring any developments to the contrary, the state of Florida will be open for business (Wednesday)."

The Disney theme parks, Universal Studios, SeaWorld Orlando and Busch Gardens in Tampa reopened Wednesday.

But all attempts to belie the horror gripping America had to face the simple, stark truths in President Bush’s morning address to the nation, in which he said America had to be on guard against attack in a "monumental struggle of good versus evil."

"It is not business as usual," he said.

Fox News' Sharon Kehnemui and the Associated Press contributed to this story.