According to John Welch, Seantum's self-incriminating admission would normally not be admissible in a Jewish court of law. Under the Talmud, no man could be put to death on his own testimony, especially in a capital case. Nevertheless, there were four exceptions: [1] Aachan (see Joshua 7); [2] The man who admitted that he had killed Saul (see 2 Samuel 1:10-16); [3] The two assassins of Ishbosheth (see 2 Samuel 4:8-12); and [4] The voluntary confession of Micah (see Judges 17:1-4). These 4 exceptions gave rise to three conditions necessary to override to 2 witness rule: [1] The self incriminating confession would need to occur outside of court; [2] God's will would need to be evidenced (prophecy, casting of lots, etc.); and [3] Physical evidence would need to be produced.
Seantum's self-incriminating confession satisfied all three of these requirements precisely: [1] Seantum immediately confessed his guilt (out of court) [2] God's will was manifest when Nephi prophesied to the people that Seantum would say specific things to the investigators when they said specific things and ask him specific questions, and [3] the investigators found blood on the skirts of Seantum's cloak.
Given the complicated and important ancient legal issues, it is especially interesting that the Book of Mormon makes special note of the fact that Seantum was "brought to prove that he himself was the very murderer" (Helaman 9:38) [John W. Welch, "The Case of an Unobserved Murder," in Reexploring the Book of Mormon, F.A.R.M.S., pp. 242-244]