Jetliners returned to the nation's skies Thursday for the first time in two days, carrying nervous passengers who faced strict new security measures following the terrorist attacks in New York and Washington.
There was tension and confusion as the aviation industry lurched back into service.
The New York area's three major airports _ Kennedy, LaGuardia and Newark, New Jersey _ were opened and then shut down as FBI officials sought a ``small number'' of individuals for questioning in the attacks and were feared to be fleeing the city. One law enforcement official, speaking on condition of anonymity, said three men were apprehended at Kennedy but other details were not released.
Earlier, a terminal at LaGuardia and Orlando International Airport were briefly evacuated over separate, unfounded bomb scares.
Elsewhere, no planes had left Chicago's busy O'Hare International Airport by late afternoon and Boston's Logan International Airport still had not met the Federal Aviation Administration's security requirements as of 5 p.m. EDT (2100 GMT).
But for the first time since Tuesday, travelers had the option of flying in the United States _ even if the service was limited to a handful of flights.
At Sky Harbor International Airport in Phoenix, Sam Hemphill was among a dozen people lined up at a TWA ticket counter. He said he was uneasy about flying but wanted to get home to Jacksonville, Florida.
``Whatever happens, happens,'' Hemphill said. ``You have to keep going. If you stop living life, they've won.''
The airlines were grounded Tuesday after jets hijacked in Boston, Newark and Washington's Dulles airport were crashed into the World Trade Center, the Pentagon and a Pennsylvania field.
Transportation Secretary Norman Mineta cleared the way for commercial flights to resume Thursday, saying airports would be opened and flights resumed on a case-by-case basis _ but only after the new security measures are in place. Private flights are still banned.
Under the tightest airport security since the Gulf War in 1991, there will be no more curbside check-ins or visits to terminal gates to meet passengers.
Knives and other cutting tools, even plastic ones, are prohibited. Mail and cargo are temporarily banned from passenger flights.
Federal marshals were on hand at many airports.
``People in this country have a hang-up about having their personal space invaded, but when you're in this situation you have to sacrifice some of your individuality,'' said Kevin McArthur, a business consultant waiting at Denver International Airport for a flight to Chicago.
The disasters were on the minds of even the most experienced travelers. Northwest Airlines employees were urged by union officials to wear black ribbons on their uniform lapels.
Some of the first flights over U.S. airspace Thursday were jets that had been diverted to Canada when the terrorists struck.
Among them was Alitalia Flight 624, which left Calgary, Alberta, for San Francisco but was forced to turn around by military jets whose pilots apparently believed American air space was off-limits to international flights. After clarification from the FAA, the flight _ which originated in Milan on Tuesday _ landed in San Francisco two hours later than expected and 48 hours late overall.
Another Alitalia flight left Calgary and landed in Los Angeles.