``The captain just said there was a national emergency and we needed to land at the nearest airport,'' Cruz said. ``We don't know what to do now.''

When told the reason for the emergency -- that hijacked planes had crashed into the World Trade Center, the Pentagon and a grassy field 80 miles from Pittsburgh -- Cruz burst into tears and walked away.

The effects of an unimaginable series of terrorist acts quickly spread across the country on Tuesday. Within hours, America was locked down.

Air traffic was halted. Tourists were turned away at popular spots, from Philadelphia's Liberty Bell to Disneyland. Air Force bases were on high alert, from Hawaii to Florida. The United Nations shut down.

Secret Service agents at the White House yelled at tourists to get away. Inside the Capitol, guards ran through the hallways shouting at people to leave.

The tunnel between Detroit and Windsor, Ontario, was closed to cars. At Newark International Airport, officers with shotguns blocked the road to the air traffic control tower.

Operators of hydroelectric dams across the West closed visitor centers and limited access.

Louisiana's Offshore Oil Port suspended tanker operations and the state's 19 oil refineries were on alert.

Security was tighter at San Francisco's Golden Gate Bridge. And while the U.S.-Mexico border crossing between San Diego and Tijuana remained open, federal agents were on security alert for terrorism.

Nuclear and weapons installations battened down, including the Department of Energy's nuclear weapons and research complex in Oak Ridge, Tenn., the Hanford nuclear reservation in Washington and the Redstone Arsenal in Alabama, site of the Army missile command.

The 192nd Virginia Air National Guard squadron of F-16 fighter jets was on alert to shoot down unauthorized aircraft.

Federal courthouses and offices nationwide were closed or put on stricter security.

Scott Coogler, a supervisor at a car rental company, was being questioned for jury service in a Miami courtroom when a marshal told prospective jurors to go home.

``The federal building, you just don't know,'' Coogler said. ``I'm on the 11th floor. Something happens, I'm toast.''

Precautions were taken at all levels of government.

California, Colorado and Illinois state government buildings were under stricter security.

Shon Hines works for a real estate company in Rockefeller Center in New York. After employees were sent home, he prepared to hike the many miles home from midtown Manhattan to the Bronx.

He wasn't thinking about the hard road ahead.

Feeling ``very vulnerable, very distressed,'' Hines said he was more concerned ``for the lives that were lost.''

A sense of vulnerability led Oklahoma City police to create a one-block perimeter around the jail where bombing conspirator Terry Nichols is housed. Nichols was convicted in the 1995 bombing that destroyed a federal building in Oklahoma City.

Space shuttle operations halted and 12,000 employees of Kennedy Space Center were sent home while surveillance was stepped up, with helicopter patrols and extra gate checks in place.

The precautions also were taken in less-obvious places.

City Hall in El Paso, Texas, closed and Denver City Hall opened an emergency preparedness office in its basement.

Greensboro College in North Carolina canceled classes and closed offices so staff and students could follow the news.

``This is history and we want them to follow it,'' spokesman Mike Clark said. ``We can't expect them to sit in class and think about differential equations while this is happening.''

The Mall of America in Bloomington, Minn., was shuttered.

Security increased at Las Vegas casinos and no visitors were allowed at St. Louis' 630-foot-tall Gateway Arch.

The Museum of Tolerance in Los Angeles was closed, as was the city's 74-story Library Tower, at 1,700 feet the tallest building west of the Mississippi.

``I was like, 'Get me out of here,''' said Angela Nalu, on her first day of a new job in the tower.