In the episode, A Day in the Strife, an alien probe approaches the station with a series of questions that have to be answered correctly within 24 hours, or Babylon 5 will be destroyed. The probe also promises knowledge of high technology to whomever answers the questions.
The probe does not in fact explode but is part of a Berserker style campaign, designed to wipe out races that are technologically superior to the creators of the probe.
Who sent the probe?
The lurker's guide said this question was not answered then, so I hope it has been answered since then. JMS did say "it was looking for civilizations that are sufficiently advanced to pose a possible threat". This seems like the kind of trick the Shadows or their allies would pull, but I don't remember any mention of that. It would also seem like the kind of trick a younger race to harm a more sophisticated race that might harm it.
Please provide answers from canon sources such as TV episodes, novels, or interviews with writers of the TV series.