The Gathering:Whodunnit?

The Pilot Episode for Babylon 5 (titled 'The Gathering')
concerns a plot to assassinate Kosh, the newly arrived ambassador from the Vorlon Empire. The assassin nearly succeeds, putting Kosh into a poison-induced coma, but through the efforts of B5 staff an antidote and the identity of the real ambassador is revealed just as a retaliatory Vorlon fleet arrives.

Through the resident telepath, Lyta Alexander, and the use of a "changeling net" the assassin frames Station Commander Sinclair for the attempted murder. Meanwhile, Narn Regime Ambassador G'kar arranges via a trial by the Babylon 5 advisory council, and by secretly contacting the Vorlon government, for Sinclair to be shipped to the Vorlon Homeworld for trial.

On the face of it there were only two conspirators in the plot: the assassin himself and a Human tech-smuggler named Varner. The assassin, a member of an exiled Minbari sect, arranged for Varner to provided him with said changeling net. He then kills Varner and succeeds in convincingly impersonating several B5 residents and staff, including Sinclair, Varner himself and Lyta Alexander by constant use of the changeling net. However, a boarding pod was discovered attached to the station's hull indicated the involvement of a third party.

It was assumed by Sinclair and his staff that the Minbari assassin arrived on a Narn transport, however the third party could easily have been rouge elements in the Earth Alliance or the Vorlons themselves. The reason for suspecting the Narns is that the Narn transport initially refused to submit to a weapon's scan then, later on, G'kar relented "in the interests of interstellar cooperation". Presumably, the boarding pod would have departed the Narn vessel after its arrival in B5 space and before G'kar changes his mind about the weapon's scan.

It was certainly reasonable for Sinclair to suspect G'kar and it was even in G'kar's interests to make Sinclair believe this to show that the Narns are not to be messed around with, but this does not mean the Narn Regime was responsible. There are several reasons not to suspect the Narns:
1. If they were responsible the entire crew of the Narn ship would all have to be conspirators and this could be considered unlikely as an increase in the number of conspirators would increase the probability of the plot being revealed prematurely.
2.G'kar was very keen to make an alliance via Delenn with the Minbari Federation. He would not jepordise this by starting a war with the Vorlons.
3.A Earth Alliance shuttle only requires one pilot and could even be put on a programmed course to and from Babylon 5. Plus, official EA ships would not be scanned for weapons as they are entitled to bring weapons aboard B5!
4.The assassin also used Takashima's palm print to access Varner's quarters. A copy of the second in command's palm print could only be obtained by a high ranking official or officer in the Earth Alliance, possibly Takashima herself.
5.If Takashima or another EA officer was involved their motive could simply be their promotion as a replacement for Sinclair would almost certainly be required if he were killed or imprisoned etc.
6.The Vorlons themselves may be invloved as they probably had the technology to revive Kosh. Also, it is strange that they would allow Sinclair to stay on the Vorlon Homeworld when they insisted Dr Kyle allow Kosh to die to protect his identity.
Reasons to suspect elements in Earth Force or the Earth Alliance:
7.Elements would certainly wish to remove Sinclair from B5 as he was percieved by some to be too close to Minbari Ambassador Delenn.
8.Certain groups advocate an isolationist stance for the EA and have resorted to violence to achieve these ends.
9.Ditto 'Home Guard' a faction which blames aliens for Earth's problems and believe aliens have too much influence in EA policy.
10.The breeching pod used by the assassin first appears moving from behind a shot of an Earth Alliance shuttle!
I think by considering the evidence and the motives the conspiracy was between the exiled Minbari sect described and high-ranking officials in Earth Force, particuarly as both sides would want to discredit Sinclair and the Minbari Government.

It is, however, entirely possible that the Vorlons arranged the whole thing as an excuse to provide a show of force, both to show that the Vorlons have considerable firepower and the will to use it and simply to avoid being dismissed as legends. Many sentients have only heard of the Vorlons but only, presumably, the Minbari have had any real contact with them so it is entirely reasonable for most sentients to dismiss the Vorlons as a fictional race, a hoax perhaps perpetuated by the Minbari themselves. Also,for reasons revealed later in the series, Sinclair would have been on of the few Humans allowed to live on Vorlon so they would be happy to keep an eye on him until needed.