On one website, a person identifying himself as a member of "The Mafia" declared: "Preparin' for the big april 20!! You'll all be sorry that day!" The date is that of Hitler's birthday, and the group were vocal admirers of Hitler.
Another message, from somebody using the name AzMstr17, warned about a war against the Power Boyz, believed to be a reference to "jocks", or sportsmen - and declares that "Columbine High School sucks". Harris and Klebold hated athletes, believing that they rode roughshod over people like them and were "mean" to them.
The "Mafia" were avid readers of the website for the "Shock Goth" singer Marilyn Manson, a 6ft 6in transvestite who sells himself as "the most evil man in America". Manson, who propagates racist and "satanist" views, has received several death threats.
Another alleged member of the "Mafia", calling himself "Asriel Abiss", stated on the Internet that his hobbies were "Listening to Manson, killing high school students, carrying guns and bombs around, hurting those who hurt me".
Yesterday investigators were examining the Internet for other messages that may have been posted by the killers ortheir friends. Such messages, they believe, may point towards accomplices in the massacre.
Tricia Primrose, a spokeswoman for the Internet service provider America Online (AOL), said: "We have no knowledge at the moment that those involved in the shooting actually posted any advanced warning on AOL. It's something we will continue to monitor."
AOL has contacted the FBI and preserved content posted by its members, in case it is tied to the massacre, but there was no evidence that anyone had posted warnings before the rampage. AOL acknowledged that hoaxes were common after big news events.
Last night the "Trenchcoat Mafia" website had been abruptly shut off, after a flurry of messages including threats of revenge for what had happened, attempts by allies to explain the gunmen's motives and others simply asking: "Why?"