Despite the biblical edict that two witnesses are required to bring testimony for evidence to be considered valid in court (Deut. 9:15), the Rabbis were lenient when it came to releasing an agunah.  Yet, according to Shiloh, “The Sages would release a woman from the state of agunah even on the basis of circumstantial evidence, and would even allow the testimony of a child, a woman, a relative, a concerned party, and even of a non-Jew speaking in good faith.  Even the agunah herself could attest to being a widow if she was the only one who saw her husband killed” (Maimonides, Hilkhot Gerushin, ch. 13).  Yoel Shiloh discusses the sources,  the justification for this leniency, and the implications in the contemporary context.