High over Canada, on a bright, late-summer morning, the crew of United Airlines Flight 945 out of Frankfurt, Germany, pointed the gleaming Boeing 747-400 south toward Chicago.

With its signature hump, the world's fastest subsonic jetliner cruises at 565 m.p.h., or about 85 percent of the speed of sound at 30,000 feet. To the 10,200 people working in Sears Tower, the jumbo jet approaching from the north Sept. 20 was as tiny and unknowable as a germ. But soon it would torment the overworked imaginations of hundreds still skittish after what happened in New York nine days before.

A few minutes before 10:30 a.m., as Flight 945 closed fast on Chicago, a flight attendant told the pilot that a woman onboard was having chest pains.

Reaching for the transponder box, either the pilot or the co-pilot dialed in the four-digit code for an emergency so the control tower would know.

But the second digit came out wrong. The code should have been 7700. Instead it came out 7500, sending the erroneous signal that a hijacker had taken over the plane and setting in motion a chain of events that quickly traversed hundreds of miles.

Before long a trader for Merrill Lynch on the floor in New York overheard a conversation on a speakerphone--a call, authorities later would say, from a passenger on Flight 945. It sounded to the eavesdropping trader like the passenger was saying the plane had been hijacked in Milwaukee and was headed for Chicago.

Quickly, the rumor from New York bounced to Chicago with a phone call to the 56th-floor offices of Merrill Lynch and spread through Sears Tower.

On the 48th floor, somebody in equities at Goldman Sachs & Co. saw a TV news report about a hijacked plane and burst into the human resources department.

"A plane from Wisconsin's been hijacked and it's headed this way," human resources worker Jodi Shivers heard her co-worker say excitedly.

For a couple of seconds, nobody spoke or moved.

Then everybody scrambled for the exits.

Outside New York and Washington and far from any field of combat, the battle joined Sept. 11 was, for most, a fierce inner struggle.

Inside the nation's tallest skyscraper, innumerable personal dramas played out, as companies pondered finding new homes and the tower's managers scrambled to make the tower more secure. This premier address, this architectural marvel with spectacular views, had become the site of a distorted reality in which lower floors were coveted and some employees prayed for layoffs. Over time, workers there, like the rest of the country, found ways to cope and settled into a new kind of normalcy.

For some of them, the hijacking scare of Sept. 20 would prove to be a turning point, informing life-changing decisions. It would persuade a securities broker on the eighth floor to keep working in the building while giving a research associate on the 70th floor one more reason to leave.

Nancy Dombrowski, a securities broker at Stifel, Nicolaus & Co. Inc., was preparing to go downstairs for a smoke when her phone rang. It was a friend and former co-worker, a stock trader at 135 S. LaSalle St.

"Did you guys hear an alarm or get an evacuation announcement?" she recalls him asking.

"What are you talking about?" Dombrowski said.

"My buddy over there, he just called saying they're evacuating because of a plane hijacking out of Milwaukee."

"No," Dombrowski said, her stomach closing into a knot.