The death of Jesus in the earliest Jesus traditions
The crucifixion of Jesus is one of the most strongly attested items in the historical Jesus inventory developed by John Dominic Crossan and which forms the basis of the Jesus Database project. This event is ranked fifth out of 522 items due to its multiple attestation:
(1) 1 Cor 15:3b; (2a) Gos. Pet. 4:10-5:16,18-20; 6:22; (2b) Mark 15:22-38 = Matt 27:33-51a = Luke 23:32-46; (2c) John 19:17b-25a,28-36; (3) Barn. 7:3-5; (4a) 1 Clem. 16:3-4 (=Isaiah 53:1-12); (4b) 1 Clem. 16.15-16 (=Psalm 22:6-8); (5a) Ign. Mag. 11; (5b) Ign. Trall. 9:1b; (5c) Ign. Smyrn. 1.2.
The two earliest written sources for the crucifixion are the authentic letters of Paul (all of which were written before the outbreak of the Jewish War, 66-73 CE), and the passion narrative in the Gospel of Mark (most likely written in the years shortly after the end of the Jewish War, even if—as Crossan suggests—it drew on an earlier account of the crucifixion that now survives in the Gospel of Peter).
Later traditions in the NT are mostly dependent on Mark and add no independent historical information to our knowledge. The one significant exception to this is the Gospel of John, which may have a good claim to preserve authentic and independent information about the death of Jesus.
See also the Passion Narrative page in the Jesus Database site.