A Minamoto samurai named Kumagai no Jiro Naozane rushed after him. He saw Atsumori and challenged him to a duel. Atsumori accepted the challenge. But because of his inexperience he was knocked to the ground.
Kumagai tore off his helmet and iron mask and then saw that his opponent was just a boy, the same age as his son. Kumagai wanted to spare Atsumori, but they were already surrounded by other Minamoto warriors; they forced Kumagai to cut off the young man's head.
Kumagai's great remorse, later recounted in the Haike Monogatari, coupled with his priestly vows, explains that this (quite common at the time) event came to light because of the great human tragedy it represented.