During a parish retreat at Tixtla, in the Mexican state of Guerrero, a sister assisting the priest opened the ciborium of consecrated hosts and saw one of them effusing a reddish substance. The host was preserved and the local bishop, Alejo Zavala Castro, opened a formal investigation.
Over four years (2009–2012), under the direction of the Pontifical Academy for Life, the relic was examined by Mexican and Italian forensic and immunological laboratories. The results, presented in 2013, identified the substance as human blood, type AB, with intact red and white blood cells of a person living at the time of analysis, and tissue consistent with cardiac muscle from the left ventricle. The bishop formally declared the event a Eucharistic miracle in October 2013.