The attack on New York's World Trade Center was nothing more or less than murder. The perpetrators voluntarily and deliberately targeted innocent civilians with their violence. And that crosses the line. The students who killed fifteen people at Columbine High School in 1999 were murderers. The men who bombed Oklahoma City's Alfred P. Murrah Federal Building in 1995 were murderers.
Serial killers are murderers.
Adolf Hitler was a murderer. Let's not dilute our descriptions of the events of September 11th, or aggrandize them with some special terminology. Let's call a spade a spade. It was mass murder, murder in the first degree. And murder demands justice.