I agree with Jack. The Johannine account, in which the Jewish authorities investigate briefly to see whiether they have enough of a case to take to Pilate, decide they do, and take the case to him, is more historically plausible than the Synoptic accounts, which lay the emphasis on Jesus' appearance before the Sanhedrin rather than on his appearance before Pilate. But there is also some literary evidece which is worth loooking at.
1) John uses _speira_("detachment"), the usual Greek equivalent of the Latin _cohors_, in Jn. 18:3,12, and _chiliarchos_, the usual Greek equivalent of the Latin _tribunus militum_ in Jn. 18:12. These words are usually used of Roman soldiers and their commmander. This involves the Romans earlier in the narrative than the Synoptics do.
2) There are some verbal parallels which link the Romans with the unbelieving "Jews." First, in Jn. 18:37 Jesus tells Pilate, "Everyone who is of the truth _akouei mou tes phones_ ("hears my voice"). This is similar phrasing to 10:27, where Jesus, describing himself as the Good Shepherd, says that his sheep _tes phones mou akouei_ ("hear my voice"). In 18:37 Jesus implicitly invites Pilate to be one of those who are of the truth; Pilate declines the invitation, as do "the Jews." Second, in Jn. 19:3 the narrator ends the description of the Roman abuse and mockery of Jesus with _edidoun autw rapismata_ ("they gave him slaps"). In Jn 18:22 Jesus receives the same treatment from a temple policeman: _edoken rapisma tw Iesou_ ("he gave Jesus a slap"). while the Gospel of John does not have a pericope of Jewish mockery of Jesus, as the Synoptics do, the narrator here clearly intends to connect the treatment of Jesus by "the Jews" and the treatment he gets from the Romans. Third, in Jn. 19:9 Pilate asks Jesus, "Where are you from?". Jesus does not answer, because a complete answer to the questiojn is an answer that Pilate cannot understand. "The Jews" also reaise the question of where Jesus is from, in Jn. 7:41b-42 and in 9:29f. But a complete answer to the question eludes them as much as it eludes Pilate. These verbal parallels link the Romans and the unbelieving "Jews" as opponents of Jesus, part of "the world," which is blind to Jesus' revelation and unwilling to receive it. I suggest that such shared characterisation is part of John's way of expressing the shared responsibility of the Romans and "the Jews" in Jesus' death.
3) We may note that all the Romans are are involved in the mistreatment and execution of Jesus. Pilate orders the flogging and actual crucifixion, and the soldiers play the game of "mock king."
4) The Johannine Pilate is a weakling and a coward. He has the authority to drop the case, especially since the Jewish authorities are not exactly forthcoming with their evidence against Jesus, and when they do speak up, their charge against Jesus is one that is outside Pilate's jurisdiction (Jn. 19:7). But he is too afraid of the Jewish authorities to stand up to them. He vacillates (expressed by his running back and forth between outside and inside, between Jesus and "the Jews"), then gives in to them when they threaten to report him to Caesar (19:12).
All in all, I suggest that while John does not let "the Jews" off the hook with respect to responsibility for Jesus' death, he does not let the Romans off the hook either.