A French man who survived the terrorist attack on the church of St. Etienne du Rouvray has described the killing of Father Jacques Hamel.
I have yet to learn at exactly what part of the mass Fr. Hamel was killed.
This New York Times source (here) says that he was killed as Mass was ending in the caption. Most sources note that he was still saying Mass.
This Catholic Answers source cites from the Catholic Herald:
The 85-year-old priest was at the altar offering the sacrifice of Mass, the moment in which the Church makes present the sacrifice of Christ’s body and blood for the salvation of the world, and his blood was literally mingled with that of Christ.
A little vague, but thanks.