Not even U.S. President George W. Bush would hazard a guess at the numbers of dead that will finally be tallied in the worst terrorist attack in American history.

Calling it a "mass murder," Mr. Bush would say only that thousands were killed.

An estimated 40,000 people worked in the towers of the World Trade Center, which collapsed after hijacked jets slammed into them yesterday morning, and roughly 24,000 work at the Pentagon, where another plane crashed.

Reports last night said at least 800 were feared dead at the U.S. military headquarters and the toll was expected to be greater in New York. At least 265 of the 400 firefighters dispatched to the site are believed to have been killed when the towers collapsed. About 85 police officers sent there are missing.

What is clearest is that 266 people aboard the flights, including the hijackers, lost their lives.

Among the dead:

Barbara Olson, 45, a lawyer and television commentator who was married to U.S. Solicitor General Theodore Olson, was on the plane that crashed at the Pentagon. She called her husband twice by cellphone during the hijacking and said the attackers were using knife-like instruments.

Mark Bingham, 31, a California public-relations worker, was on the plane that went down near Pittsburgh. He called his mother on an airplane phone just before the crash and told her "we've been taken over by three men who say they have a bomb." Alice Hoglan, his mother, said he told her: "I love you very much, in case I don't see you again." She said there was "a lot of commotion in the background when he called."

Garnet (Ace) Bailey, director of pro scouting for the Los Angeles Kings, was the first Canadian confirmed dead. The 53-year-old was on the second plane that crashed into the World Trade Center.