"We don't appear to be targeted, but we cannot be too careful," says Mayor Richard M. Daley. "We have to take every precaution necessary, but you cannot panic people."

By mid-morning, the Illinois Department of Transportation opened reversible lanes of expressways out of the Central Business District, at least four hours earlier than normal, while Chicago Transit Authority and Metra trains were headed out of Downtown immediately after loading.

The first building evacuation involved the 110-story Sears Tower. Management of the Aon Center, John Hancock Building, Merchandise Mart and Wrigley Building followed suit. All have massive amounts of office space, but the Hancock building at 875 N. Michigan Ave. also includes condominiums.

Chicago Police Supt. Terry Hillard says closures are up to individual building owners and managers as well as tenants. He adds the police has worked with the Building Owners and Management Association on security issues.

"They know what we know," Hillard says. "We've tried to get them to harden their targets. Folks cannot walk in and out of buildings unchallenged now."

The attack at the nation's financial center also prompted evacuations at the Chicago Board of Trade, Chicago Mercantile Exchange and Chicago Stock Exchange in the South and West Loop even before the Security and Exchange Commission shut down operations at all financial markets.

Shopping centers to close included Water Tower Place and nearby Shops at 900 N.Michigan. Suburban centers also closed, including Chicago Ridge Mall, Fox Valley Shopping Center, Louis Joliet Mall, North Riverside Mall, Oakbrook Center and Stratford Square. Opting to stay open throughout the morning was Old Orchard in north suburban Skokie, before it closed shortly after noon.

Government buildings closed included the Daley Center, home of city offices; Thompson Center, which houses retail space as well as state government offices; Dirksen Federal Building as well as the criminal courts complex at 26th Street and California Avenue.

Daley says the city's water stations and pumping stations were on lockdown, and freight tunnels that run underground in the Central Business District were being closed and secured. However, Chicago public schools remained open Tuesday, while most Downtown colleges and universities closed.

Chicago Fire Commissioner James Joyce says his firefighters would have approached a high-rise catastrophy the same way his counterparts who valiantly tried to rescue victims of the World Trade Center attack did.

"My thoughts immediately went out to the New York Fire Department and Police Department, because I'm sure they had a lot of people in those buildings that collapsed," Joyce says. "You go to work from inside the building. That's what firefighters do every day."

The Field Museum was the first public building to close, which was immediately followed by the Shedd Aquarium, Terra Museum and Art Institute.

In Downstate Springfield, the State Capitol was evacuated.