"Those of us who value our liberty, even in the face of danger, will need to be vigilant in the days to come," says Thomas Leavitt, an online activist who co-founded Webcom.

Other civil libertarians say it's a mistake to believe that the U.S. government will overreact to Tuesday's disasters. Marc Rotenberg of the Electronic Privacy Information Center said he believes that the better approach is to argue that the U.S. must not allow a terrorist attack on our form of open government to succeed.

It's too early to tell whether he's right or not, but by late Tuesday, operators of anonymous remailers were already so worried about being conduits for terrorist communications -- or being blamed for the communications, rightly or wrongly -- that they pulled the plug.

Operator Len Sassaman said in a post to a remailer-operators list: "I don't want to get caught in the middle of this. I'm sorry. I'm currently unemployed and don't have the resources to defend myself. At this point in time, a free-speech argument will not gain much sympathy with the Feds, judges and general public."

Remailers forward messages but remove the originating information, so that the resulting e-mail is anonymized. They customarily don't keep logs, so if the system works as designed, it should be nearly impossible for anyone to find who sent the message.